What is email deliverability?

Email deliverability is the ability of your emails to reach the recipient’s inbox rather than being filtered into spam.

In simple terms, deliverability works like a trust system. The more consistently you send wanted, engaged-with emails, the more inbox access you earn. The more negative signals you generate, the more that access shrinks.

Email delivery vs email deliverability

An email is considered delivered when the receiving mail server accepts it. That simply means it didn’t bounce. But delivered does not mean seen. An email can be accepted by a server and still:

• Land in the spam folder
• Be filtered into promotions.
• Be deprioritized or hidden from view.

Deliverability measures where the message actually lands, not just whether it was accepted.

Why does email deliverability matter?

Email deliverability has a direct impact on visibility, engagement, and revenue. If emails don’t reach the inbox, they might as well not exist. For marketing teams, poor deliverability leads to:

• Lost revenue and missed conversions
• Wasted campaign budget
• Unreliable performance data
• Wrong strategic decisions about messaging

You might think your messaging is weak when the real issue is that 15–20% of your audience never saw the email. But deliverability doesn’t just affect marketing performance. For SaaS companies, the operational impact is even bigger:

• Activation emails in spam reduce onboarding completion.
• Password reset emails in spam create support tickets.
• Filtered out billing emails delay payments.
• Security alerts that don’t reach users increase risk exposure.

Over time, the impact compounds. When engagement drops, filtering becomes stricter, further reducing visibility and performance. What starts as a small inbox placement issue can quietly turn into a sustained growth problem. When emails don’t consistently reach the inbox, growth slows, operations suffer, and customer relationships weaken.

That’s why monitoring deliverability proactively is essential.

How is email deliverability measured?

Email deliverability is not shown as a single universal score inside most ESP dashboards. Instead, it’s assessed using inbox placement data and supporting performance metrics.

The primary metric used to measure deliverability is inbox placement rate — the percentage of delivered emails that land in the inbox rather than spam.

The formulas are:

Delivery rate = (Emails delivered ÷ Emails sent) × 100
Inbox placement rate = (Emails in inbox ÷ Emails delivered) × 100

For example:

• 10,000 emails sent
• 9,500 delivered
• 8,500 land in the inbox

Delivery rate: 95%
Inbox placement rate: 89.5%

A high delivery rate does not guarantee strong deliverability. An email can be accepted by the receiving server but still filtered into spam.

How inbox placement is measured

Mailbox providers do not provide direct inbox placement data for every recipient. As a result, deliverability is estimated using:

• Seed list testing (sending emails to controlled inboxes across major providers)
• Dedicated deliverability testing tools that generate inbox placement reports

Supporting metrics that influence deliverability

Because inbox placement cannot be directly observed for every recipient, teams monitor related signals that correlate with filtering decisions:

• Bounce rate – Percentage of emails that fail to deliver
• Spam complaint rate – Percentage of recipients who mark emails as spam
• Unsubscribe rate – Percentage opting out
• Engagement rate (opens and clicks) – How recipients interact with emails
• Domain and IP reputation – Sender reputation maintained by mailbox providers

Inbox placement is the primary measure of deliverability, but these metrics help explain why deliverability rises or falls.

What does “good” email deliverability look like?

Inbox placement rates vary by industry and by email type. Marketing and transactional emails are filtered differently, so their benchmarks are different too.

Marketing emails (newsletters, promotions, campaigns): For marketing emails, global inbox placement benchmarks typically fall in the 83–85% range, based on data from Validity. Here’s a practical way to think about your numbers:

• Below 80% → Likely reputation, engagement, or list quality issues. Needs attention.
• 80–89% → Acceptable, but performance can be improved.
• 90%+ → Strong sender reputation and healthy engagement.

Transactional emails (password resets, receipts, alerts): Transactional emails usually perform better. They’re expected, time-sensitive, and often generate stronger engagement.

Healthy transactional senders commonly see high inbox placement (often in the mid-90s or above). If you’re consistently below about 95%, it may point to authentication gaps, infrastructure issues, or broader reputation concerns. Properly configured transactional emails should rarely be filtered.

Provider differences matter: Inbox placement also varies by mailbox provider. For example, Gmail may show different filtering behavior compared to Microsoft-hosted services like Outlook or Microsoft 365, which are often stricter.

Ultimately, good deliverability isn’t about hitting a number once; it’s about maintaining consistent inbox placement over time, performing reliably across providers, and meeting expectations for the type of email you send.

Factors that affect email deliverability

Deliverability comes down to a few moving parts: your technical setup, how you send emails, and how people respond to them. All of these work together.

Technical authentication

Let’s start with the foundation: authentication.

Authentication tells mailbox providers that your emails are actually coming from you and that they haven’t been spoofed or tampered with along the way. Without it, your messages do not just risk going to spam. They can be rejected outright.

Today, proper authentication is not optional. Mailbox providers like Gmail and Yahoo expect bulk senders to have SPF, DKIM, and DMARC correctly configured. If these are not set up properly, your emails are far more likely to be filtered or blocked.

There are three core components:

• SPF: Specifies which mail servers are authorized to send email on behalf of your domain, helping receiving servers detect unauthorized sources.
• DKIM: Adds a cryptographic signature to each message so receiving servers can verify that the content has not been modified during transmission.
• DMARC: Builds on SPF and DKIM by enforcing alignment with the visible From domain and defining how authentication failures should be handled, while also providing reporting for monitoring.

For strong results, SPF, DKIM, and DMARC should all pass and align with your sending domain. If they do not, or if DMARC is configured too loosely, it can weaken trust in your domain and reduce deliverability.

Repeated authentication failures do not just affect a single campaign. Over time, they can damage your domain reputation and make recovery more difficult, especially at scale.

Sending infrastructure and reputation signals

Beyond authentication, mailbox providers also evaluate your sending infrastructure. They look at a range of technical and behavioral signals, including:

• IP reputation and complaint history: Providers track spam complaints, bounce patterns, and recipient engagement tied to a specific sending IP over time. A poor history at the IP level can affect every message sent from it.
• Domain reputation and sending history: Your domain builds its own track record. Past abuse signals, inconsistent sending practices, or sudden behavioral changes can all influence long-term trust scoring.
• Reverse DNS (PTR) and forward-confirmed reverse DNS: Properly configured PTR records that map back to your sending domain help confirm that your mail server is legitimate and correctly set up.
• HELO or EHLO hostname identity: The hostname presented during the SMTP handshake should be valid, resolvable, and aligned with your infrastructure. Mismatches can raise red flags.
• TLS encryption during transmission: Using STARTTLS for secure transport signals a properly configured and security-conscious sending environment.
• Sending volume consistency: Gradual growth and predictable sending patterns build trust over time. Sudden spikes, erratic volume, or large one-time blasts often trigger filtering or throttling.
• Tracking and link domain alignment: Branded, aligned tracking or redirect domains appear more trustworthy than generic or mismatched third-party domains.
• Blacklist status and abuse signals: Active listings on major blocklists or unresolved abuse activity can significantly restrict inbox placement until addressed.
• Shared versus dedicated IP environment: In shared IP setups, your deliverability can be influenced by other senders using the same IP. Dedicated IPs provide full control over reputation, but also full responsibility for maintaining it.

Email content and structure

Once authentication and infrastructure checks pass, mailbox providers evaluate the message itself. At this stage, filtering systems analyze both what recipients see and what sits behind the scenes in the code.

This goes beyond wording. Providers assess formatting, HTML structure, link behavior, and consistency across elements to determine whether a message appears legitimate, promotional, or potentially deceptive.

Common evaluation signals include:

• Deceptive or misleading subject lines: Subject lines that exaggerate claims, misrepresent the content, or create false urgency can reduce trust and increase the likelihood of spam classification.
• Excessive promotional or urgency-based language: Frequent use of aggressive sales phrases, all caps, or artificial scarcity messaging can raise filtering risk, particularly when combined with other negative signals.
• Broken or suspicious links: Invalid URLs, excessive redirects, or links pointing to low-reputation domains can trigger security and spam filters.
• Large image-to-text ratios: Image-heavy emails with very little readable text may appear evasive to filters and can negatively affect deliverability.
• Poorly structured or invalid HTML: Sloppy markup, missing tags, or unoptimized code from visual editors can create rendering inconsistencies and raise filtering concerns.
• Hidden text or obfuscated content: Invisible text, extremely small fonts, or code designed to conceal content are strong spam indicators.
• Mismatch between From name, subject line, and body content: Inconsistencies between sender identity and message intent can resemble phishing behavior and reduce trust.

It is important to understand that filtering systems evaluate patterns rather than isolated elements. A single promotional phrase is unlikely to cause filtering on its own. However, when multiple high-risk signals appear within the same message, the probability of spam classification increases significantly.

Recipient engagement and behavioral signals

Beyond technical setup and content, mailbox providers pay close attention to how recipients interact with your emails. Engagement signals help them decide whether your future messages belong in the inbox, the promotions tab, or the spam folder.

They typically evaluate signals such as:

• Open rates and read time: Consistent opens and meaningful read duration suggest that recipients expect and value your emails.
• Click activity: Genuine clicks on relevant links indicate positive intent. In contrast, erratic or bot-like click patterns may trigger additional scrutiny.
• Replies and forwards: Direct replies and message forwarding are strong trust signals, showing that real people are engaging with your content.
• Messages moved from spam to inbox: When users manually mark your email as “not spam,” it sends a corrective positive signal to the provider.
• Messages deleted without being opened: Frequent unopened deletions suggest low relevance and can gradually weaken future inbox placement.
• Spam complaints: When recipients mark messages as spam, providers treat this as a high-severity negative signal that can quickly damage sender reputation.
• Unsubscribes: While unsubscribes are normal at healthy levels, sudden spikes may indicate misaligned expectations or poor targeting.
• Long-term inactivity: Continuously sending to subscribers who never open or click lowers overall engagement ratios and can drag down domain reputation over time.

These engagement signals accumulate and form a behavioral profile of your sending patterns.

List quality and acquisition practices

List quality shapes the audience you send to and directly affects the risk profile of your campaigns. Even with strong authentication and content, poor list practices can quickly undermine deliverability.

Mailbox providers pay close attention to how addresses are collected and maintained. Key factors include:

• Permission-based acquisition: Collecting addresses through explicit opt-in ensures recipients knowingly agree to receive communication, which reduces future complaint risk.
• Clear and transparent subscription flows: It should be obvious what users are signing up for, how often they will hear from you, and what type of content they will receive. Misaligned expectations often lead to complaints or disengagement.
• Confirmed opt-in when appropriate: Adding a confirmation step helps validate address ownership and reduces fake, mistyped, or bot-generated submissions.
• Bounce handling and removal: Hard bounces should be automatically removed to prevent repeated delivery attempts to invalid or non-existent addresses.
• Suppression of long-term inactive contacts: Periodically reviewing and suppressing contacts who have not engaged over extended periods helps reduce list decay and protects engagement ratios.
• Avoidance of purchased or scraped lists: Third-party lists often contain outdated data, recycled addresses, spam traps, or individuals who never granted consent, all of which significantly increase filtering risk.
• Monitoring for spam trap exposure: Spam traps are designed to identify poor acquisition practices. Hitting them can severely damage domain and IP reputation.

Tips for improving email deliverability

The checklist below is a quick recap of the factors we just covered, highlighting the core practices that support strong inbox placement.

Here are a few additional tips to help strengthen deliverability further.

Protect your most important email streams: Use separate subdomains for marketing and transactional emails. This prevents campaign performance (such as lower engagement or higher complaints) from affecting critical system messages like password resets and receipts.

Warm up new domains and IPs gradually: When using a new domain or dedicated IP, start with low sending volumes and increase steadily over several weeks. Warm-up periods typically last about 4–8 weeks, depending on your overall volume and engagement.

Keep spam complaint rates very low: Industry benchmarks generally indicate that keeping spam complaint rates below 0.1% per campaign (1 complaint per 1,000 delivered emails) helps minimize reputation risk. Monitoring complaint trends over time is more important than focusing on a single send.

Maintain healthy bounce rates: For permission-based lists, bounce rates are typically expected to remain under a few percent. Higher bounce rates may indicate outdated data, acquisition issues, or list hygiene problems. Hard bounces should be removed immediately.

Prioritize engaged recipients: Engagement signals such as opens, clicks, replies, and spam complaints influence future inbox placement. Sending to recently engaged subscribers first can help reinforce positive performance signals, particularly during new campaigns or list reactivation efforts.

Review performance across providers regularly: Inbox placement can vary by mailbox provider. Periodic testing helps ensure that messages are reaching the inbox consistently and allows you to identify shifts in filtering behavior before they impact overall results.

Tools for testing email deliverability

Most ESPs show delivery and engagement performance, but they do not provide inbox placement visibility or full cross-provider reputation data. For that, teams rely on dedicated deliverability and monitoring tools.

Testing before sending (technical and message validation)

Before a campaign goes live, teams validate authentication, DNS configuration, and message structure. Tools like MXToolbox verify SPF, DKIM, and DMARC records, while Google Admin Toolbox helps inspect full email headers to see how authentication was applied to a specific message.

For content and spam risk analysis, platforms like Mail Tester evaluate emails against common filtering rules, many of which are based on systems like SpamAssassin, flagging potential issues in formatting, headers, links, or wording that may increase filtering risk.

Many teams also run end-to-end tests during development or QA. Tools like testmail.app provide controlled inboxes where messages can be captured and reviewed, including authentication results, spam scores, HTML structure, links, attachments, and trigger timing.

Testing where emails actually land (inbox placement)

To understand real-world placement, teams use seed-based inbox testing. Platforms such as GlockApps, Validity Everest, and Unspam maintain networks of test addresses across major mailbox providers. By sending campaigns to those accounts, they estimate whether emails land in the inbox, spam folder, or other tabs.

It’s important to note that many modern deliverability platforms overlap across categories. For example, tools like Unspam often combine inbox placement testing with authentication checks, spam analysis, and reputation monitoring in a single solution.

Monitoring sender health over time

Deliverability also requires ongoing monitoring. While your email service provider tracks engagement metrics such as opens, clicks, unsubscribes, bounces, and spam complaints, this is only part of the picture.

Mailbox-provider reputation data is typically available through external tools. Platforms like Google Postmaster Tools and Microsoft SNDS provide insight into domain and IP reputation, complaint rates, and filtering signals directly from the providers themselves.

How often should you test email deliverability?

Email deliverability should be reviewed regularly, especially when changes occur in your sending setup or performance. A good approach is to test:

• Before launching a new campaign, product flow, or major send, so you can confirm authentication, content quality, and inbox placement before scaling volume.
• After infrastructure changes, such as switching ESPs, updating DNS records, adding a new sending domain, or moving to a new IP. These changes can directly affect reputation and filtering.
• When engagement starts to decline, since falling open or click rates can sometimes signal inbox placement issues or shifts in audience behavior.
• After significant content updates, including new templates, changes in subject line strategy, or adjustments in sending frequency, as these can influence filtering systems.
• On a regular schedule, many teams run monthly or quarterly checks to monitor reputation, engagement trends, and inbox placement before small issues grow into larger problems.