How to improve email deliverability: A practical guide 

You can spend hours crafting the perfect subject line, building personalized customer journeys, and designing beautiful emails. But if your messages don't make it to the inbox, all that work goes to waste.

That's why email deliverability deserves just as much attention as the content of your campaigns. Deliverability is what determines whether your emails reach your customers, get filtered into spam, or disappear before they're ever seen.

The good news is that improving email deliverability isn't about finding a single trick or setting. It's about consistently following best practices that help mailbox providers trust you as a sender. When you build that trust, you're more likely to reach the inbox, engage customers, and get better results from every email you send.

In this guide, we'll explain what email deliverability is, what influences it, and the practical steps you can take to improve it over time. Whether you're sending onboarding emails, lifecycle campaigns, transactional messages, or newsletters, these best practices will help more of your emails land where they belong.

Email deliverability is simply the successful delivery of emails to your recipients' inboxes rather than their spam folders or other filtered locations.

It's easy to confuse email delivery with email deliverability, but they're not the same thing.

Delivery means that a receiving mail server accepted your email. It doesn't tell you where the message ended up. An email can be successfully delivered to a mailbox provider but still be routed to spam, the promotions tab, or another filtered folder where it's unlikely to be read.

Deliverability, on the other hand, measures whether your email reaches the inbox where your audience is most likely to see and engage with it.

Mailbox providers like Gmail, Yahoo, Outlook, and Apple Mail evaluate every incoming message before deciding where it belongs. They consider hundreds of signals, including your sender reputation, authentication records, sending history, recipient engagement, and the content of your emails.

No email service provider can guarantee inbox placement because those decisions ultimately belong to the receiving mailbox provider. What you can control is how trustworthy you appear as a sender. Following deliverability best practices helps you build and maintain that trust over time.

Strong deliverability has a direct impact on every stage of the customer lifecycle.

If your welcome email lands in spam, new customers may never complete onboarding. If password reset emails are delayed or filtered, customers become frustrated, and support tickets increase. If promotional campaigns miss the inbox, engagement drops, and revenue follows.

In other words, better deliverability helps ensure that the messages you work hard to create actually reach the people they're intended for.

Deliverability also affects your sender reputation. Mailbox providers continuously evaluate how recipients interact with your emails. When people open your messages, click links, reply, or move emails out of spam, those positive signals reinforce that you're sending valuable content. When recipients ignore your emails, delete them without reading, or mark them as spam, your reputation can decline.

Because sender reputation builds over time, improving deliverability isn't something you do once and forget. It's an ongoing process that rewards consistent sending practices and relevant, engaging content.

For teams using Customer.io, deliverability is especially important because lifecycle messaging depends on timely, reliable communication. Whether you're welcoming new customers, encouraging product adoption, announcing new features, or sending critical transactional emails, reaching the inbox is essential to creating the customer experience you intended.

There's no single factor that determines inbox placement. Instead, mailbox providers look at a combination of technical configuration, sending behavior, and recipient engagement to decide whether your emails deserve a place in the inbox.

Some of the biggest factors include:

• Your sender reputation and sending history.
• Whether your domain is properly authenticated with SPF, DKIM, and DMARC.
• The quality of your email list.
• How recipients engage with your emails.
• How consistently you send emails over time.
• Whether your emails generate spam complaints or high bounce rates.

Think of deliverability as a trust score that's constantly being updated. Every campaign either strengthens or weakens that trust, depending on recipients' responses.

The rest of this guide walks through each of these factors in more detail and shares practical ways to improve your email deliverability without resorting to shortcuts or outdated tactics.

Every email you send contributes to your reputation as a sender. Mailbox providers don't judge campaigns in isolation. Instead, they look at patterns over time to determine whether recipients find your emails useful and trustworthy.

Some of those signals are technical, like whether you've authenticated your sending domain. Others are behavioral, like whether people open your emails, click your links, or mark your messages as spam. Together, they help mailbox providers decide whether your next campaign belongs in the inbox.

While every provider uses its own algorithms, a handful of factors consistently influence deliverability.

Think of your sender reputation as your email credit score. It's a measure of how trustworthy mailbox providers believe you are, based on your sending history and recipients' interactions with your emails.

A strong sender reputation tells providers that people want to receive your messages. A poor reputation suggests the opposite, making it more likely that future emails will be filtered or blocked.

Mailbox providers don't publish a single reputation score, and each one evaluates senders differently. But many of the underlying signals are similar, including:

• Spam complaint rates
• Bounce rates
• Recipient engagement
• Send volume and consistency
• The age and reputation of your sending domain and IP address

The important thing to remember is that reputation is earned over time. One successful campaign won't instantly improve a poor reputation, just as one mistake usually won't destroy a healthy one. Consistently sending relevant emails to engaged audiences is what builds long-term trust.

Email authentication helps prove that messages sent from your domain are legitimate and haven't been spoofed or altered in transit.

Without authentication, mailbox providers have a harder time verifying your identity. That uncertainty increases the likelihood that your emails will be filtered or rejected.

Three authentication standards form the foundation of modern email security:

SPF (Sender Policy Framework) identifies which mail servers are authorized to send email on behalf of your domain.

DKIM (DomainKeys Identified Mail) adds a digital signature that allows receiving servers to verify that your email hasn't been modified after it was sent.

DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds on SPF and DKIM by telling mailbox providers how to handle emails that fail authentication checks. It also provides reporting that helps you identify authentication issues and protect your domain from abuse.

These standards work together to establish trust with mailbox providers and protect your brand from phishing attempts. They're no longer considered optional for organizations that rely on email to communicate with customers.

Even the best email program will struggle if it's built on a poor-quality contact list.

Healthy email lists are made up of people who knowingly subscribed, want to hear from you, and continue to engage with your messages. Unhealthy lists contain invalid addresses, abandoned inboxes, inactive subscribers, or contacts that never asked to receive your emails in the first place.

Over time, every email list naturally degrades. People change jobs, abandon old email accounts, or lose interest in certain brands. That's why list hygiene should be an ongoing practice rather than an occasional cleanup project.

Sending to outdated or disengaged contacts increases the likelihood of:

• Hard and soft bounces
• Spam complaints
• Low engagement rates
• Spam trap hits
• A declining sender reputation

Purchased or rented email lists create even greater risks. Because recipients haven't explicitly opted in to hear from you, they're more likely to ignore your emails or report them as spam. Many purchased lists also contain invalid addresses or spam traps that can significantly damage your reputation.

A smaller, engaged audience almost always outperforms a larger list filled with inactive contacts.

Mailbox providers pay close attention to how recipients interact with your emails because engagement is one of the strongest indicators of whether your messages provide value.

Positive engagement signals include actions like:

• Opening emails
• Clicking links
• Replying to messages
• Moving emails out of the spam folder
• Adding your sending address to a contact list

Negative signals include deleting emails without reading them, consistently ignoring your messages, or marking them as spam.

While open rates have become less reliable as a standalone metric because of privacy features like Apple Mail Privacy Protection, mailbox providers have access to many additional engagement signals that marketers can't directly measure. That's why it's more important than ever to send emails that recipients genuinely want to receive instead of focusing on individual metrics.

Relevance is one of the biggest drivers of engagement. When you send messages based on customer behavior, preferences, and lifecycle stage, recipients are naturally more likely to interact with them. Those positive interactions reinforce your sender reputation over time.

Mailbox providers expect legitimate senders to establish predictable sending patterns.

Large spikes in email volume, sudden changes in sending frequency, or long periods of inactivity followed by massive campaigns can all look suspicious. Those patterns resemble the behavior of compromised accounts or spammers, which may prompt additional filtering.

Consistency doesn't mean you need to send an email every day. It means your sending volume should generally align with your normal patterns.

If you're introducing a new sending domain or dramatically increasing volume, it's usually better to ramp up gradually. This process, often called warming a domain or IP, gives mailbox providers time to observe positive engagement before trusting larger campaigns.

For established senders, maintaining a regular cadence also helps set expectations with subscribers. When customers know how often they'll hear from you, they're less likely to be surprised by your emails or report them as spam.

Many marketers assume certain words automatically trigger spam filters. Years ago, that was often true. Today, mailbox providers take a much more sophisticated approach.

The content of your email still plays a role, especially if it contains misleading subject lines, broken links, excessive imagery, or formatting that resembles spam. But content is just one signal among many.

An authenticated sender with a strong reputation can often use language that would have raised concerns years ago without harming deliverability. On the other hand, even well-written emails may struggle to reach the inbox if they're sent from a poorly authenticated domain with a history of low engagement.

Rather than trying to outsmart spam filters, focus on creating emails that recipients genuinely want to receive. Relevance, trust, and consistency are much more effective long-term strategies than chasing outdated spam trigger lists.

The good news is that most of the factors that influence deliverability are within your control. By building healthy audiences, authenticating your domain, sending consistently, and prioritizing customer relevance, you create a strong foundation for inbox placement.

At its core, deliverability is a cycle. The more consistently you earn your subscribers' trust, the more positive engagement signals mailbox providers see. That stronger reputation leads to better inbox placement, creating more opportunities to engage customers and continue building trust.

The flywheel below illustrates how these elements work together.

Next, we'll look at the practical steps you can take to improve email deliverability and maintain that foundation over time.

There's no quick fix for email deliverability. The best results come from consistently following practices that build trust with mailbox providers and create better experiences for your customers.

The good news is that most of these practices are well within your control. Whether you're launching a new email program or looking to improve an established one, these twelve recommendations will help more of your emails reach the inbox.

Before you send a single campaign, make sure your sending domain is properly authenticated.

Authentication allows mailbox providers to verify that emails sent from your domain are legitimate. It also protects your brand from spoofing and phishing attempts while improving the trustworthiness of your messages.

At a minimum, you should configure:

• SPF to authorize the servers that can send email on your behalf.
• DKIM to digitally sign your messages.
• DMARC to define how mailbox providers should handle emails that fail authentication.

Authentication has become a baseline expectation for modern email programs. If you haven't configured these records yet, it's one of the highest-impact improvements you can make. Get ramped up by reviewing Google's email sender requirements or Yahoo's sender best practices.

Mailbox providers are naturally cautious about new sending domains. If a brand-new domain suddenly starts sending hundreds of thousands of emails, it can resemble the behavior of a spammer.

Instead, build trust over time.

Start with your most engaged subscribers, send smaller volumes, and gradually increase your sending as mailbox providers observe positive engagement. This process, often called warming your domain, helps establish a healthy sending reputation from the beginning.

The same principle applies if you're dramatically increasing your normal sending volume. Gradual growth is almost always better than sudden spikes.

Permission is one of the strongest foundations of good deliverability.

People should understand what they're signing up for, what kinds of emails they'll receive, and how often you'll contact them. Setting those expectations early reduces surprises later.

It's also important to honor those expectations. If someone subscribes to receive product updates, they probably aren't expecting daily promotional emails.

Some organizations choose to use double opt-in to further confirm subscriber intent. While it may reduce list growth slightly, it can also improve engagement by ensuring that subscribers genuinely want to hear from you.

No matter which subscription process you use, avoid purchased or rented email lists. They often contain outdated addresses, disengaged recipients, or spam traps that can quickly damage your sender reputation.

One of the easiest ways to improve engagement is to make your emails more relevant.

Rather than sending every campaign to your entire list, segment your audience based on customer behavior, preferences, lifecycle stage, or product usage. The more relevant a message feels, the more likely recipients are to open it, click it, and continue engaging with future emails.

For example, you might send:

• Welcome emails to new customers.
• Product education to users exploring a specific feature.
• Re-engagement campaigns to inactive subscribers.
• Upgrade announcements to customers approaching usage limits.

Behavioral messaging naturally creates better customer experiences because people receive timely, useful information rather than generic messages.

It's also good for deliverability. Higher engagement signals help reinforce your reputation with mailbox providers over time.

Every email list becomes less accurate over time.

People change jobs, abandon old email addresses, unsubscribe mentally without clicking the unsubscribe link, or simply lose interest in a product they once loved.

That's why list hygiene should be an ongoing part of your email program.

Review your audience regularly and remove or suppress contacts who consistently bounce, never engage, or have become inactive over long periods. Before suppressing inactive subscribers, consider running a re-engagement campaign to give them one final opportunity to stay subscribed.

While it may feel counterintuitive to reduce your audience, a smaller list of engaged subscribers typically performs better than a much larger list of inactive contacts.

Bounce rates offer valuable insight into the health of your email program.

There are two primary types of bounces:

• Hard bounces occur when an email can't be delivered because the address is invalid, doesn't exist, or has permanently stopped accepting mail.
• Soft bounces are temporary failures caused by issues like a full inbox, a temporary server problem, or a message that's too large.

A few soft bounces are normal. Consistently high bounce rates, especially hard bounces, often indicate poor list quality or outdated subscriber information.

Monitoring bounce trends can help you identify problems early, before they affect your sender reputation.

It might seem contradictory that encouraging unsubscribes can improve deliverability, but it's true.

When people no longer want your emails, giving them an easy way to opt out is much better than forcing them to mark your messages as spam.

Every spam complaint sends a strong negative signal to mailbox providers. An unsubscribe, on the other hand, simply tells you that a subscriber's preferences have changed.

Make your unsubscribe link easy to find, honor requests promptly, and consider offering a preference center where subscribers can adjust the types or frequency of emails they receive instead of leaving altogether.

Respecting customer preferences builds trust, even when someone decides it's time to unsubscribe.

For years, marketers treated open rates as the primary measure of email success. Today, that metric tells only part of the story.

Privacy features, including Apple Mail Privacy Protection, have made opens less reliable indicators of genuine engagement.

Instead, look at a broader set of signals, including:

• Click-through rates.
• Replies.
• Conversions.
• Website activity after email clicks.
• Long-term engagement trends.

The goal isn't to maximize any single metric. It's to consistently send emails that customers find valuable enough to interact with.

When recipients regularly engage with your emails, mailbox providers receive positive signals that boost future inbox placement.

Consistency helps mailbox providers understand what normal looks like for your organization.

If you typically send a weekly newsletter, continue doing so on a predictable schedule. If your lifecycle campaigns are triggered by customer behavior, ensure they're operating consistently and reliably.

Sudden bursts of email after months of inactivity can raise concerns, particularly if recipients aren't expecting to hear from you.

A regular sending cadence also helps build trust with subscribers. When customers know what to expect, they're more likely to continue engaging with your emails.

Small mistakes can have an outsized impact on both customer experience and deliverability.

Before launching a campaign, verify that:

• Links work correctly.
• Images load as expected.
• Personalization renders properly.
• Authentication is functioning.
• The email displays well across devices and clients.

Testing won't guarantee inbox placement, but it helps eliminate avoidable issues that could reduce engagement or create a poor experience for recipients.

Deliverability isn't something you measure only after a problem appears.

Regularly monitor indicators like:

• Spam complaint rates.
• Bounce rates.
• Delivery rates.
• Domain authentication reports.
• Inbox placement trends.

Tools like Google Postmaster Tools and DMARC reports can provide additional visibility into how mailbox providers view your sending domain.

Watching these metrics consistently makes it easier to identify gradual changes before they become major deliverability issues.

The most successful email programs don't think about deliverability only when something goes wrong.

Instead, they build healthy habits into their everyday workflows.

They regularly clean their lists, review engagement metrics, monitor authentication, respect subscriber preferences, and continually optimize their messaging based on customer behavior.

Deliverability isn't a finish line you cross once. Mailbox providers continuously evaluate your reputation, and customer expectations continue to evolve. Treating deliverability as an ongoing practice helps ensure your emails continue reaching the inbox long after your next campaign.

Ultimately, improving deliverability comes down to earning trust. When you consistently send relevant emails to people who want them, make it easy to manage preferences, and follow modern sending best practices, both your customers and mailbox providers are more likely to reward you.

If you're looking for a quick way to evaluate your email program, use this checklist as a starting point.

• Authenticate your sending domain with SPF, DKIM, and DMARC.
• Warm up new sending domains and gradually increase sending volume.
• Send email only to people who have opted in to receive it.
• Segment your audience so emails are relevant to each recipient.
• Regularly remove or suppress inactive subscribers.
• Monitor hard and soft bounce rates.
• Make it easy for subscribers to unsubscribe or manage their preferences.
• Look beyond open rates and track meaningful engagement metrics.
• Maintain a consistent sending cadence.
• Test campaigns before sending.
• Monitor your sender reputation and authentication reports.
• Review your deliverability practices regularly and make improvements over time.

Improving deliverability doesn't require mastering every recommendation overnight. Start with the fundamentals, monitor your results, and build better habits over time. Small improvements made consistently often have a much bigger impact than one-time fixes.

Even the savviest marketers occasionally run into deliverability problems. In many cases, the root cause isn't a single technical issue but a series of small decisions that gradually erode sender reputation.

Some of the most common mistakes include:

• Sending emails without properly authenticating your domain.
• Purchasing or renting email lists.
• Continuing to send to inactive subscribers who never engage.
• Sending large, unexpected spikes in email volume.
• Ignoring bounce rates or spam complaints.
• Making it difficult for subscribers to unsubscribe.
• Sending the same message to every customer regardless of their interests or behavior.

If any of these sound familiar, don't panic. Your sender reputation can improve over time when you learn and adopt healthier sending practices and consistently provide value to your subscribers.

No platform can guarantee inbox placement. Mailbox providers ultimately decide where each email belongs.

What your customer engagement platform can do is make it easier to follow the practices that support strong deliverability.

Customer.io helps teams send more relevant emails by making it easy to build audiences based on customer behavior, lifecycle stage, product usage, and other first-party data. Instead of sending the same campaign to every subscriber, you can deliver timely, personalized messages that encourage stronger engagement over time.

Customer.io also supports healthy audience management with suppression lists, subscription preferences, and flexible segmentation tools that help you target the right people with the right message.

As your program grows, reporting and analytics make it easier to monitor campaign performance, identify engagement trends, and continuously refine your messaging strategy. Combined with support for domain authentication and reliable infrastructure, these capabilities help you build an email program that's designed for long-term success.

Ultimately, deliverability isn't about finding a platform that promises inbox placement. It's about choosing tools that help you build trust with your audience and consistently follow email best practices.

There's no universal benchmark because deliverability depends on factors like your industry, audience, and sending practices. In general, healthy email programs maintain high delivery rates while also monitoring inbox placement, engagement, bounce rates, and spam complaints. Looking at these metrics together provides a much more accurate picture than any single number.

Emails can be routed to spam for many reasons, including missing authentication records, poor sender reputation, high spam complaint rates, low recipient engagement, inconsistent sending patterns, or poor list quality. Improving deliverability usually means addressing several of these factors rather than looking for one specific cause.

Yes. Configuring SPF, DKIM, and DMARC helps mailbox providers verify that your emails are legitimate. While authentication alone won't guarantee inbox placement, it's a foundational best practice that supports sender trust and protects your domain from spoofing.

It depends on the underlying issue and the health of your sender reputation. Some improvements, like adding authentication, can have an immediate impact. Others, such as rebuilding sender reputation after high complaint rates or poor list hygiene, may take weeks or months of consistently following best practices.

In most cases, yes. Subscribers who never engage can lower overall engagement rates and weaken your sender reputation over time. Before removing them entirely, consider running a re-engagement campaign to give them an opportunity to remain subscribed.

Email deliverability isn't just a technical challenge. It's the result of every interaction you have with your subscribers.

When you send relevant emails to people who want to hear from you, authenticate your domain, maintain healthy lists, and respect subscriber preferences, you're building trust with both your audience and mailbox providers.

That trust compounds over time. Better engagement strengthens your sender reputation, a stronger sender reputation improves inbox placement, and better inbox placement creates more opportunities to build meaningful customer relationships.

Deliverability isn't something you achieve once and check off a list. It's an ongoing practice that should evolve alongside your customers, your products, and the email ecosystem itself.

By focusing on relevance, consistency, and customer experience, you'll put your email program in the best possible position to reach the inbox and drive results.