In the competitive landscape of digital communication, simply hitting 'send' isn't enough. The real challenge lies in navigating the complex web of spam filters, ISP algorithms, and sender reputation scores to ensure your message actually reaches its intended recipient. This is the core of email deliverability: the art and science of getting your emails into the primary inbox, not the spam folder or, worse, blocked entirely.

Poor deliverability directly translates to wasted effort, missed revenue opportunities, and a tarnished brand reputation. Every email that fails to land is a potential conversation lost. For small businesses, sales teams, and marketing professionals, mastering this discipline is non-negotiable for achieving campaign goals and maximizing return on investment.

This comprehensive guide is designed to provide a clear, actionable roadmap. We will move beyond generic advice and dive deep into the most critical email deliverability best practices that distinguish successful campaigns. You will learn the specific technical setups, strategic list management techniques, and content optimizations required to build trust with internet service providers and connect directly with your audience.

Here, you will find a detailed breakdown of the essential pillars of deliverability, including:

• Authentication Protocols: Implementing SPF, DKIM, and DMARC correctly.
• List Hygiene: Maintaining a clean and engaged subscriber base.
• Sender Reputation: Proactively managing your sending scores.
• Content Optimization: Crafting messages that bypass spam filters.

By implementing these strategies, you can transform your email outreach from a shot in the dark to a reliable, direct line to your customers, ensuring your campaigns achieve the impact they deserve.

1. Authentication Protocols (SPF, DKIM, DMARC)

Think of email authentication protocols as your domain's digital passport. They prove to mailbox providers like Gmail and Outlook that your emails are legitimately from you, not from a spoofer or phisher impersonating your brand. This verification is a foundational element of any effective email deliverability best practices strategy, directly impacting whether your messages land in the inbox or the spam folder.

Without proper authentication, you're essentially an anonymous sender, making it easy for inbox providers to treat your messages with suspicion. Implementing these protocols is non-negotiable for building a strong sender reputation.

How Authentication Works: A Three-Layered Approach

Email authentication relies on three core protocols that work together to verify your sender identity. Each plays a distinct role:

• Sender Policy Framework (SPF): This is the first layer. SPF allows you to create a list of approved IP addresses and servers authorized to send email on behalf of your domain. You publish this list as a TXT record in your Domain Name System (DNS). When an email arrives, the receiving server checks the SPF record to see if the sending IP is on your approved list.
• DomainKeys Identified Mail (DKIM): This adds a second layer of security through a digital signature. DKIM attaches an encrypted, invisible header to every email you send. The receiving server uses a public key, also stored in your DNS, to decrypt this signature. If the keys match, it proves the email's content hasn't been tampered with in transit.
• Domain-based Message Authentication, Reporting & Conformance (DMARC): This is the final and most powerful layer. DMARC tells receiving servers what to do if an email fails either SPF or DKIM checks (or both). It also provides detailed reports on all email activity for your domain, including legitimate and fraudulent attempts.

This infographic illustrates the recommended, sequential process for implementing these crucial email deliverability best practices.

This step-by-step flow highlights how each protocol builds upon the last to create a comprehensive authentication framework.

Actionable Implementation Tips

Implementing authentication can seem technical, but a phased approach simplifies it. Work with your IT team or domain provider to configure these DNS records.

1. Start with SPF: Create your SPF record listing all services you use to send email (e.g., Google Workspace, Mailchimp, your CRM).
2. Add DKIM: Next, generate and add DKIM keys for each of your sending platforms.
3. Deploy DMARC Gradually: Begin with a "monitor-only" policy (p=none). This allows you to receive reports without impacting email delivery. Review these reports to ensure your legitimate emails are passing authentication.
4. Enforce DMARC: Once you're confident, move to a stricter policy like p=quarantine (sends failures to spam) or p=reject (blocks them entirely).

2. List Hygiene and Segmentation

Think of your email list as a garden. To thrive, it needs regular weeding and care. List hygiene is the process of cleaning your list by removing inactive subscribers and invalid addresses, while segmentation involves grouping your subscribers based on their behavior or interests. This combination is a cornerstone of advanced email deliverability best practices, ensuring your messages reach an engaged audience that wants to hear from you.

Sending emails to a stale or unengaged list is a direct path to the spam folder. Mailbox providers monitor how recipients interact with your emails. High bounce rates, low open rates, and spam complaints signal that your content is unwanted, which severely damages your sender reputation.

This ongoing maintenance ensures you communicate only with subscribers who are genuinely interested, boosting engagement and protecting your deliverability. For a deeper dive into the maintenance process, you can learn more about effective email list cleaning strategies.

How List Hygiene and Segmentation Work

These two practices work in tandem to improve campaign performance and maintain a healthy sender score. They create a positive feedback loop with inbox providers.

• List Hygiene: This is the reactive, cleaning part of the process. It involves regularly identifying and removing certain types of contacts from your active sending lists. This includes hard bounces (invalid or non-existent email addresses), soft bounces that persist over time, and long-term inactive subscribers who haven't opened or clicked an email in a significant period (e.g., 6-12 months). This reduces negative signals sent to mailbox providers.
• Segmentation: This is the proactive, strategic part. Instead of sending a generic message to everyone, you divide your audience into smaller, more focused groups. Segments can be based on demographics (location, age), purchase history, website behavior (pages visited), or engagement level (frequent openers vs. at-risk subscribers). This allows you to send highly relevant content that resonates with each group, increasing positive engagement signals like opens and clicks.

For example, an e-commerce brand like Airbnb segments users by their travel preferences and past booking history to send targeted destination guides. Similarly, a B2B company like HubSpot might automatically suppress contacts who have been inactive for over six months, preserving their sender reputation.

Actionable Implementation Tips

Integrating list hygiene and segmentation into your email strategy is a continuous effort, not a one-time task. Here is a practical approach:

1. Automate Bounce Management: Configure your email service provider (ESP) to immediately remove hard bounces. For soft bounces, set a rule to remove them after three consecutive failed deliveries.
2. Implement a Sunset Policy: Create a re-engagement or "win-back" campaign for subscribers who haven't engaged in 3-6 months. If they don't respond, move them to a suppression list.
3. Segment by Engagement: Create dynamic segments for your most engaged users (opened/clicked in 30 days), moderately engaged (30-90 days), and at-risk (90+ days). Prioritize sending to your most engaged segment, especially during critical campaigns.
4. Use Double Opt-In: From the very beginning, require new subscribers to confirm their email address. This prevents typos and spam traps from entering your list and ensures you have a high-quality, engaged audience from day one.

3. Sender Reputation Management

Think of your sender reputation as your domain's credit score. Internet Service Providers (ISPs) like Gmail and Microsoft use it to decide how trustworthy you are. A high score means your emails are welcomed into the inbox, while a low score gets you sent directly to the spam folder or blocked entirely. Actively managing this score is a cornerstone of any successful email deliverability best practices strategy.

Your reputation is built over time based on your sending habits. Inconsistent volumes, high complaint rates, or sending to invalid addresses can quickly damage it. Without a strong reputation, even the most well-crafted emails will never reach their audience, making consistent monitoring and maintenance non-negotiable.

How Sender Reputation is Scored

ISPs calculate your reputation using a variety of data points. They monitor the behavior associated with your sending IP addresses and domains to determine if you are a responsible sender. Each provider has its own algorithm, but they generally focus on the same core metrics:

• Sending Volume & Consistency: Sudden, massive spikes in email volume are a red flag for ISPs. Consistent, predictable sending patterns signal a legitimate sender.
• Complaint Rates: This is the percentage of recipients who mark your email as spam. A high complaint rate is one of the most damaging factors to your reputation.
• Bounce Rates: A high hard bounce rate (emails sent to invalid or non-existent addresses) indicates poor list hygiene and negatively impacts your score.
• Engagement Metrics: ISPs notice when recipients open, click, and reply to your emails. Positive engagement builds your reputation, while a lack of it can hurt you over time.

For example, a company like Salesforce leverages sophisticated reputation monitoring to automatically throttle sending if negative signals are detected, protecting the overall health of its sending infrastructure. Similarly, Mailchimp provides users with dashboards to track their own reputation metrics against benchmarks.

Actionable Implementation Tips

Proactively managing your reputation is much easier than repairing a damaged one. Follow these steps to build and maintain a positive sender score.

1. Warm-Up New IPs and Domains: Never send high volumes from a new IP address or domain immediately. Gradually increase your sending volume over several weeks to build a positive history with ISPs.
2. Maintain Consistent Sending Habits: Avoid erratic sending schedules. Aim for a stable, predictable volume and frequency to establish a trustworthy pattern.
3. Monitor Your Reputation Score: Use tools like Validity's Sender Score or Google Postmaster Tools to track your reputation. Check these scores regularly to catch and address issues before they escalate.
4. Address Negative Signals Immediately: If you see a spike in bounces or spam complaints, act fast. Pause your campaigns, clean your email list, and identify the root cause of the problem.

For a deeper understanding of building your reputation from scratch, you can learn more about the IP and domain warm-up process on plusvibe.ai, which is a critical first step in reputation management.

4. Content Optimization and Spam Filter Avoidance

Beyond technical setup, the content of your email itself is a primary factor in whether you reach the inbox. Spam filters are sophisticated algorithms that analyze every part of your message, from the subject line to the body copy and HTML code, looking for red flags. Optimizing your content is a crucial email deliverability best practice that ensures your carefully crafted message doesn't get flagged as junk mail.

Even with perfect authentication and a stellar sender reputation, poorly designed content can land you in the spam folder. Inbox providers prioritize user experience, and messages that appear spammy, irrelevant, or deceptive are quickly filtered out.

How Content Quality Impacts Deliverability

Spam filters assess numerous content-related signals to score your email's "spamminess." These signals help them decide if your email is a valuable communication or unwanted clutter. Ignoring these signals is a direct path to the junk folder.

• Keyword Triggers: Certain words and phrases are classic indicators of spam. Think "free money," "act now," or "risk-free." Modern filters are smarter than ever, but overusing aggressive sales language can still trigger them. Brands like The Skimm excel by using a conversational, news-like tone that avoids robotic spam patterns and feels more personal.
• Formatting and Code: The way your email is built matters. Excessive use of capital letters, multiple exclamation points, and brightly colored fonts are all classic spam characteristics. Likewise, sloppy HTML code or relying on a single, large image with little to no text is a major red flag for filters.
• Relevance and Personalization: The most effective way to stay out of the spam folder is to send content people want to read. Personalized and relevant messages are less likely to be marked as spam by recipients, which sends positive signals to inbox providers. Grammarly balances its promotional emails with genuinely valuable writing tips, keeping subscribers engaged and deliverability high.

Actionable Implementation Tips

Proactively cleaning up your content before you hit "send" will dramatically improve your inbox placement rates. Focus on creating a clean, professional, and user-friendly experience.

1. Balance Your Text-to-Image Ratio: Aim for a ratio of at least 60% text to 40% images. An email that is just one large image is a huge spam trigger because filters can't read the content.
2. Avoid Spammy Formatting: Keep your subject lines and body copy professional. Use capital letters and exclamation points sparingly and only when necessary for emphasis.
3. Prioritize Clean HTML: Use clean, simple HTML for your templates. Broken tags or unnecessarily complex code can be flagged by spam filters.
4. Always Include a Visible Unsubscribe Link: Hiding your unsubscribe link frustrates users and encourages them to mark your email as spam. Make it easy to find in your footer.
5. Test Before You Send: Before launching a campaign, run your email through a testing tool. You can learn more about using a cold email spam checker to identify potential issues and optimize your content for better delivery.

5. Permission-Based Marketing and Double Opt-in

Building a high-quality email list starts with earning explicit permission, not just collecting addresses. Permission-based marketing is the ethical and strategic practice of only sending emails to individuals who have clearly and knowingly agreed to receive them. This foundational approach, often facilitated by a double opt-in process, is one of the most impactful email deliverability best practices you can implement.

When every subscriber on your list has actively chosen to be there, you are guaranteed a more engaged audience. This directly signals to mailbox providers like Gmail and Outlook that your content is valuable and wanted, significantly boosting your sender reputation and keeping your messages out of the spam folder.

How Permission and Double Opt-in Work

This strategy focuses on obtaining and verifying consent, creating a list of genuinely interested recipients. It's a two-step confirmation that ensures list quality from the very beginning.

• Single Opt-in: A user enters their email address into a signup form and is immediately added to your list. While this creates a low-friction experience, it's vulnerable to typos, spam traps, and people signing up others without their consent.
• Double Opt-in: This adds a crucial verification step. After a user signs up, they receive an automated confirmation email and must click a link inside it to be added to your list. This confirms the email address is valid and that the owner genuinely wants to subscribe. Platforms like ConvertKit and Mailchimp strongly advocate for this method, often setting it as the default for new accounts.

For example, the popular newsletter Morning Brew uses clear opt-in language and a seamless confirmation process. This ensures subscribers are engaged from day one, which is key to their massive open rates and strong deliverability.

Actionable Implementation Tips

Implementing a double opt-in system is straightforward and yields long-term benefits for your email program. The goal is to make the process clear, fast, and rewarding for the new subscriber.

1. Use Clear Opt-in Language: Be explicit on your signup forms. Tell people what they are subscribing to and how often they can expect to hear from you (e.g., "Get our weekly marketing tips every Tuesday").
2. Send Confirmation Emails Immediately: Trigger the confirmation email the moment a user submits your form. Delays can lead to subscribers forgetting they signed up, causing them to ignore the confirmation or, worse, mark future emails as spam.
3. Provide a Strong Value Proposition: Clearly articulate the benefit of subscribing. Whether it’s exclusive content, a discount, or valuable insights, make the "why" compelling.
4. Launch a Welcome Series: Once a subscriber confirms their email, engage them immediately with a welcome series. This reinforces the value of their subscription and trains them to open your emails.

6. Consistent Sending Patterns and Frequency

Imagine a new friend who only contacts you at random, unpredictable times, sometimes bombarding you with messages and then disappearing for months. You'd likely start ignoring their calls. Mailbox providers like Gmail and Outlook view erratic senders in a similar way. Establishing a regular, predictable email sending schedule is a cornerstone of effective email deliverability best practices, signaling to both subscribers and Internet Service Providers (ISPs) that you are a reliable and trustworthy sender.

Inconsistency is a major red flag for spam filters. Sudden, massive spikes in email volume from a domain that is usually quiet can look like a compromised account or a spam attack. By maintaining a steady rhythm, you build a dependable sender reputation, which helps your messages consistently reach the inbox.

How Consistency Builds Trust

Predictability is key to building trust with ISPs. They use historical data to create a baseline for your typical sending behavior. Sticking to that baseline proves your legitimacy over time.

• Consistent Volume: Sending roughly the same number of emails per day or week helps ISPs recognize your normal activity. A sudden jump from sending 1,000 emails a week to 100,000 in a day will trigger alarms.
• Consistent Frequency: Sticking to a regular cadence, whether daily, weekly, or bi-weekly, aligns with subscriber expectations and reinforces your pattern with ISPs. Brands that master this build an appointment with their audience. For instance, Morning Brew’s daily 6 AM EST newsletter is a prime example of a brand building a powerful habit with its readers through sheer consistency.
• Consistent Timing: Sending at the same time of day helps establish a pattern. If you always send your newsletter on Tuesdays at 10 AM, ISPs learn to expect it, and your subscribers do too.

This predictable behavior is a powerful signal that you are a legitimate marketer, not a spammer exploiting a domain.

Actionable Implementation Tips

Adopting a consistent sending schedule requires strategy and discipline. It's about finding a rhythm that works for your audience and your operations.

1. Survey Your Audience: The best way to know how often people want to hear from you is to ask. Use a simple survey to ask subscribers their preferred frequency (daily, weekly, monthly).
2. Start Small and Scale: If you are a new sender or are re-engaging a cold list, begin with a lower frequency and smaller volume. Gradually increase it as your engagement metrics stay positive, allowing ISPs to adjust to your new patterns.
3. Implement a Preference Center: Empower your subscribers by giving them control. A preference center allows users to choose what types of content they receive and how often, reducing unsubscribes and increasing engagement.
4. Automate Your Schedules: Use your email marketing platform to schedule campaigns in advance. This ensures you maintain your day-of-week and time-of-day patterns without manual intervention, preventing accidental deviations that could harm your deliverability.

7. Engagement Monitoring and Re-engagement Campaigns

Think of your email list as a garden. To keep it healthy, you must not only plant new seeds but also tend to the existing plants, pruning away the ones that no longer thrive. Engagement monitoring is this essential gardening practice, a core component of any advanced email deliverability best practices strategy. It involves tracking how subscribers interact with your emails to identify who is active and who has become dormant.

Sending emails to a list full of unengaged contacts signals to mailbox providers like Gmail and Outlook that your content isn't relevant or wanted. This actively harms your sender reputation, increasing the likelihood that all your emails, even those sent to your most loyal fans, end up in the spam folder.

How Engagement Monitoring Works: A Data-Driven Approach

The process involves systematically tracking key metrics to segment your audience based on their activity levels. This allows you to treat different subscriber groups appropriately, preserving the health of your overall list.

• Define Your Metrics: The most common engagement metrics are opens and clicks. More advanced tracking can include conversions, replies, or time spent reading. Defining what an "engaged" subscriber looks like for your business is the first step.
• Set an Inactivity Threshold: Establish a clear timeframe to classify a subscriber as inactive. A common standard is a lack of any opens or clicks within a 90 to 180-day period (3-6 months). This window can be adjusted based on your sending frequency and business cycle.
• Segment and Target: Once you identify this inactive segment, you can launch a targeted re-engagement or "win-back" campaign designed specifically to rekindle their interest. Subscribers who still don't respond after this effort should be removed from your active sending lists.

This proactive management demonstrates to inbox providers that you are a responsible sender committed to maintaining a high-quality, engaged audience.

Actionable Implementation Tips

Turning monitoring into action is what protects your sender reputation. A structured re-engagement strategy can recover valuable subscribers while safely pruning those who are gone for good.

1. Craft Compelling "Win-Back" Content: Create a dedicated 2-3 email sequence for your inactive segment. Use subject lines that create curiosity or a sense of loss, such as "Is this goodbye?" or "We miss you, [First Name]."
2. Offer a Clear Incentive: Give them a reason to click. This could be an exclusive discount, a free resource, a sneak peek at a new product, or an invitation to update their email preferences.
3. Make the Call-to-Action Unmistakable: The goal is to get a click. Use a clear, single call-to-action (CTA) like "Yes, Keep Me on the List!" or "Claim My 25% Off." This makes it easy for them to signal their renewed interest.
4. Know When to Let Go: If a subscriber doesn't engage with your win-back campaign, it's time to remove them. Continuing to email them will only damage your deliverability. Respect their silence and confidently purge them from your active lists.

By implementing these tactics, you can improve engagement metrics, which is a crucial first step. If you want to dive deeper into boosting engagement further, you can explore strategies to get a 20% reply rate to your cold emails. Learn more about advanced engagement techniques on plusvibe.ai.

8. Technical Infrastructure and Monitoring

Your technical infrastructure is the engine powering your email program. It encompasses everything from the servers and IP addresses you send from to the systems that monitor performance. A robust, well-maintained infrastructure signals to mailbox providers like Gmail and Yahoo that you are a serious, professional sender, which is a critical component of any email deliverability best practices strategy.

Investing in the right technical setup provides control, stability, and crucial data insights. Without it, you are flying blind, unable to diagnose issues, react to blocking events, or scale your sending volume effectively. High-volume senders, in particular, cannot achieve consistent inbox placement without a solid technical foundation.

How Technical Infrastructure Works: The Core Components

A high-performing email infrastructure relies on several interconnected elements. Each plays a specific role in ensuring your messages are sent efficiently and accepted by receiving servers.

• Email Service Provider (ESP): Professional ESPs like SendGrid or Amazon SES provide the core sending infrastructure, including servers, software, and established relationships with Internet Service Providers (ISPs). Their expertise and scale are invaluable. For instance, Slack leverages Amazon SES for its massive volume, building custom monitoring on top of its reliable foundation.
• Dedicated IP Addresses: For high-volume senders, a dedicated IP address is essential. This means your emails are the only ones sent from that IP, so its reputation is entirely under your control. This is a contrast to shared IPs, where your deliverability can be affected by other senders.
• Real-Time Monitoring and Alerting: This involves tracking key metrics like delivery rates, bounce rates, spam complaint rates, and open/click rates as they happen. An effective system will automatically alert you to sudden negative changes, allowing for rapid intervention.
• Feedback Loops and Bounce Handling: These are automated processes for managing unsubscribes, spam complaints (via feedback loops), and bounces. Proper configuration ensures you promptly remove problematic addresses from your list, protecting your sender reputation.

Actionable Implementation Tips

Building a strong technical foundation is a strategic process. Focus on selecting the right partners and configuring systems for maximum visibility and control.

1. Choose a Reputable ESP: Select an ESP known for strong deliverability performance and good ISP relationships, such as SendGrid, Mailgun, or Amazon SES.
2. Use Dedicated IPs for Volume: If you send over 100,000 emails per month, invest in a dedicated IP address to isolate and build your own sender reputation.
3. Implement Real-Time Monitoring: Set up dashboards and alerts to track your deliverability metrics. If your ESP's native tools are limited, use third-party services to gain deeper insights.
4. Configure Bounce and Complaint Handling: Ensure your system automatically processes hard bounces, soft bounces, and spam complaints from feedback loops to maintain list hygiene.
5. Test Your Setup Regularly: Don't assume everything is working perfectly. You must actively test email deliverability to identify and fix issues before they impact your entire program.

8-Key Email Deliverability Practices Comparison

From Sending to Connecting: Your Path to Deliverability Mastery

Navigating the complex world of email deliverability can feel like a daunting task, but it doesn't have to be. Throughout this guide, we've unpacked the essential pillars that support a successful email strategy, moving from abstract concepts to concrete, actionable steps. The journey from the "send" button to a subscriber's inbox is paved with technical precision, strategic foresight, and a genuine commitment to your audience. Mastering these email deliverability best practices is not about finding a single magic bullet; it's about building a resilient, trustworthy system piece by piece.

Think of it as constructing a bridge. Your technical authentication protocols, SPF, DKIM, and DMARC, are the foundational pylons driven deep into the bedrock. They provide the structural integrity that mailbox providers demand. Without them, everything else is unstable. Your meticulously maintained and segmented email lists are the strong, reliable steel beams, ensuring that only engaged and valid recipients are part of the structure.

Building on that foundation, your sender reputation acts as the smooth, well-paved road surface. Every positive interaction, every open, and every click adds to its quality, while bounces and spam complaints create damaging potholes. This is where consistent sending schedules and IP warming come into play, gradually building trust and demonstrating reliability to Internet Service Providers (ISPs).

Synthesizing Strategy into Sustainable Success

The content of your emails is the traffic that flows across this bridge. Optimizing your messages to avoid spam triggers and provide genuine value ensures a smooth, continuous flow. Permission-based marketing and double opt-in processes are the traffic signals and clear signage, guiding only interested parties onto the bridge and preventing congestion from uninterested subscribers.

Finally, vigilant monitoring of your engagement metrics and technical infrastructure acts as your ongoing maintenance crew. This team constantly inspects the bridge for signs of wear, patches up small issues before they become major problems, and uses re-engagement campaigns to guide straying traffic back into the main flow. Each of these eight components works in concert. A weakness in one area, such as poor list hygiene, will inevitably strain the others, compromising the entire structure.

The ultimate goal of focusing on these email deliverability best practices is to transform your perspective. You'll stop seeing email as a one-way broadcast and start viewing it as a two-way conversation built on a foundation of trust. This shift is what separates campaigns that get lost in the digital noise from those that build lasting customer relationships, drive meaningful conversions, and deliver a powerful return on investment.

Your Actionable Roadmap to the Inbox

To begin your journey toward deliverability mastery, focus on a phased approach. Don't try to implement everything at once. Start with a thorough audit:

1. Technical Foundation First: Use a free online tool to check your SPF, DKIM, and DMARC records. Correcting these is your highest-priority task, as it's the gatekeeper for everything else.
2. Immediate List Cleanup: Run your primary email list through a validation service. Removing invalid, high-risk, and unengaged contacts will provide an immediate boost to your sender reputation.
3. Review Your Content: Analyze the subject lines and body content of your last five campaigns. Identify and eliminate any common spam trigger words or deceptive formatting, such as excessive capitalization or misleading "Re:" prefixes.

By tackling these three areas first, you address the most critical factors influencing how ISPs perceive your emails. From there, you can systematically work through optimizing your sending frequency, implementing re-engagement flows, and monitoring your performance over time. This methodical approach turns an overwhelming checklist into a manageable, strategic plan for sustainable email success.

Ready to automate the technical heavy lifting and focus on what you do best? PlusVibe integrates domain warming, real-time email verification, and advanced deliverability analytics into one seamless platform. Stop guessing and start connecting by exploring how PlusVibe can ensure your essential messages always reach the inbox.