Email Deliverability: What It Is & How to Improve It

You could write the perfect email, design a beautiful template, and target exactly the right audience. But if your email lands in the spam folder, none of it matters. Email deliverability is the foundation that everything else in email marketing is built on, and yet it is one of the most misunderstood and overlooked topics.

This guide explains what email deliverability actually is, what factors influence it, and provides a practical playbook for improving it. Whether you are troubleshooting deliverability problems or proactively building a solid sending reputation from scratch, this is your reference.

What Is Email Deliverability?

Email deliverability is the ability of your emails to reach the inbox (not the spam folder, not the promotions tab, but the primary inbox). It is different from email delivery, which simply measures whether the email was accepted by the recipient’s mail server.

An email can be “delivered” (accepted by Gmail’s servers) but still end up in spam. Deliverability is about where the email ends up after delivery.

The Key Metrics

• Delivery rate: Percentage of emails accepted by the mail server (not bounced). Healthy is 95%+
• Inbox placement rate: Percentage of delivered emails that reach the inbox (not spam). This is the real deliverability metric. Healthy is 85%+
• Bounce rate: Percentage of emails rejected. Hard bounces (invalid addresses) should be below 2%. Soft bounces (temporary issues) should be below 5%
• Spam complaint rate: Percentage of recipients who mark your email as spam. Must stay below 0.1% (1 in 1,000). Google and Yahoo enforce this threshold strictly
• Unsubscribe rate: Not a deliverability metric directly, but high unsubscribe rates signal content relevance problems that eventually affect deliverability

The Three Pillars of Deliverability

Email deliverability rests on three interconnected pillars: authentication, reputation, and content. Weakness in any one area undermines the others.

Pillar 1: Email Authentication

Authentication proves to receiving mail servers that you are who you say you are, and that your emails have not been tampered with in transit. There are three authentication protocols you need to set up, and they are all required.

SPF (Sender Policy Framework)

SPF is a DNS record that lists which mail servers are authorized to send email on behalf of your domain. When Gmail receives an email claiming to be from your domain, it checks your SPF record to verify the sending server is on the approved list.

How to set it up: Add a TXT record to your domain’s DNS. Your email marketing tool will provide the specific value to include. It typically looks like:

v=spf1 include:_spf.mailerlite.com ~all

You can include multiple senders in one SPF record, but be aware of the 10 DNS lookup limit. If you exceed it, SPF validation fails.

DKIM (DomainKeys Identified Mail)

DKIM adds a digital signature to your emails that receiving servers use to verify the message was not altered in transit and truly came from your domain. It involves adding a public key to your DNS and having your email tool sign outgoing messages with the corresponding private key.

How to set it up: Your email marketing tool generates a DKIM key pair. You add the public key as a DNS TXT record (usually a CNAME pointing to the tool’s DNS). The tool handles signing automatically.

DMARC (Domain-based Message Authentication, Reporting & Conformance)

DMARC tells receiving servers what to do with emails that fail SPF or DKIM checks, and where to send reports about authentication failures. It is the policy layer on top of SPF and DKIM.

Start with a monitoring policy to collect data without affecting delivery:

v=DMARC1; p=none; rua=mailto:dmarc@yourdomain.com

After monitoring for a few weeks and confirming your legitimate emails pass authentication, gradually tighten the policy to p=quarantine and eventually p=reject.

Pillar 2: Sender Reputation

Your sender reputation is a score that ISPs (Gmail, Outlook, Yahoo, and others) assign to your sending IP address and domain based on your historical sending behavior. It is similar to a credit score: built slowly over time and damaged quickly by bad behavior.

IP Reputation vs. Domain Reputation

Historically, IP reputation was the primary factor. If you sent from a clean IP, your emails reached the inbox. But ISPs have shifted increasingly to domain reputation, meaning your sending domain’s history matters more than ever.

This is relevant because when you use a shared email marketing platform, you share IP addresses with other senders. Your deliverability is partly influenced by their behavior. This is one reason why reputable email tools strictly enforce anti-spam policies and can suspend accounts that damage shared IP reputation.

Dedicated IPs give you full control over your IP reputation but require sufficient sending volume (typically 50,000+ emails per month) to maintain a warm IP. Most small and mid-size senders are better off on shared IPs from a reputable provider.

How to Build and Maintain Reputation

• Send consistently. Erratic sending patterns (nothing for months, then a massive blast) look suspicious to ISPs
• Warm up new IPs and domains gradually. Start with small volumes to your most engaged subscribers and increase over weeks
• Remove hard bounces immediately. Continuing to send to invalid addresses damages reputation fast
• Honor unsubscribes promptly. Legal requirement and reputation essential
• Keep spam complaints below 0.1%. This is the single most important metric for reputation

Checking Your Reputation

Google Postmaster Tools is the best free resource for monitoring your domain reputation with Gmail. It shows your domain and IP reputation, spam rate, authentication success, and encryption status. If you send significant volume, set this up immediately.

Pillar 3: Content and Engagement

ISPs increasingly use engagement signals to determine inbox placement. If recipients open, click, reply to, and forward your emails, ISPs learn that your emails are wanted. If recipients ignore, delete, or mark your emails as spam, ISPs learn the opposite.

Content Best Practices for Deliverability

Subject lines: Avoid all-caps, excessive punctuation, and classic spam trigger words. This does not mean you cannot use the word “free” in a subject line (that myth is outdated), but context matters. A subject line that reads like spam to a human will likely trigger spam filters too.

HTML quality: Use clean, well-structured HTML. Avoid broken tags, excessive images with minimal text, and extremely large file sizes. The text-to-image ratio should lean toward text.

Links: Do not include too many links in a single email. Avoid URL shorteners (especially bit.ly and similar services that are frequently abused by spammers). Use your own domain for links whenever possible.

Unsubscribe: Make it easy and obvious to unsubscribe. A visible unsubscribe link reduces spam complaints because recipients choose to unsubscribe rather than hitting the spam button. Include the List-Unsubscribe header so email clients can display their own unsubscribe button.

A Practical Deliverability Improvement Playbook

If you are experiencing deliverability issues, or you want to proactively build a strong foundation, follow this step-by-step process.

Step 1: Audit Your Authentication

Verify that SPF, DKIM, and DMARC are properly configured for every domain you send from. Use a tool like MXToolbox or Mail Tester to check your records. Fix any issues before moving on.

Most email marketing platforms walk you through authentication setup. Postmark and MailerLite make this especially straightforward, with clear instructions and verification checks built into their setup flow.

Step 2: Clean Your List

A dirty list is the most common cause of deliverability problems. Clean it aggressively:

• Remove hard bounces: These should be automatically handled by your email tool, but verify
• Remove inactive subscribers: Anyone who has not opened or clicked in 90-180 days should go through a re-engagement sequence (see our automation guide) and then be removed if they do not re-engage
• Verify old or imported lists: If you are importing a list or have not sent in a while, run it through an email verification service first. Sending to a stale list full of invalid addresses is one of the fastest ways to destroy your reputation
• Check for spam traps: Email verification services can identify potential spam trap addresses. Hitting spam traps is a strong negative signal to ISPs

Step 3: Warm Up Properly

If you are sending from a new domain or IP, or if you are resuming sending after a long pause, warm up gradually:

Week 1: Send to your most engaged 500-1,000 subscribers only Week 2: Expand to your most engaged 2,500-5,000 subscribers Week 3: Expand to 10,000-15,000 Week 4: Full list (if list is under 50,000)

For larger lists, continue the ramp-up over 6-8 weeks. The goal is to demonstrate positive engagement signals (opens and clicks) before ISPs see large volumes from your domain.

Step 4: Segment by Engagement

Create segments based on subscriber engagement levels:

• Highly engaged (opened or clicked in last 30 days): Send to these subscribers first and most frequently
• Moderately engaged (opened or clicked in last 31-90 days): Send regularly but not as aggressively
• Low engagement (no opens or clicks in 91-180 days): Send less frequently, focus on re-engagement content
• Inactive (no engagement in 180+ days): Run through a re-engagement sequence, then remove

This segmentation strategy ensures that your most engaged subscribers, who generate positive signals for ISPs, receive your emails first.

Step 5: Monitor Continuously

Set up ongoing monitoring:

• Google Postmaster Tools for Gmail reputation data
• Bounce and complaint tracking in your email tool’s dashboard
• Seed testing (sending to test accounts at various ISPs) to spot inbox placement issues before they affect your whole list
• Blocklist monitoring to catch if your IP or domain lands on a major blocklist

Tools with the Best Deliverability

While deliverability depends more on your practices than your platform, some tools invest more in infrastructure, compliance, and support for high inbox placement.

Postmark

Postmark is built specifically for deliverability. They maintain separate infrastructure for transactional and marketing emails, enforce strict content policies, and publish their deliverability metrics publicly. If inbox placement is your top priority, Postmark is the gold standard. They are particularly strong for transactional emails (order confirmations, password resets, receipts) where delivery speed and reliability are critical.

The trade-off is that Postmark is deliberately opinionated. They do not allow cold outreach, have strict content guidelines, and their marketing features are less extensive than full-featured marketing platforms. This strictness is what protects their deliverability.

MailerLite

MailerLite consistently ranks among the best marketing platforms for deliverability. They enforce strict anti-spam policies, provide easy authentication setup, and maintain clean shared IP pools. For a general-purpose marketing tool, their deliverability is excellent.

ActiveCampaign

ActiveCampaign offers strong deliverability backed by good infrastructure and compliance practices. Their predictive sending feature, which sends emails at the optimal time for each individual subscriber, can improve engagement metrics and indirectly help deliverability. For advanced marketers who want automation depth alongside good deliverability, ActiveCampaign is a strong choice.

SendGrid

SendGrid is an infrastructure-focused platform owned by Twilio. It is widely used for both transactional and marketing email, with granular deliverability tools including dedicated IP management, IP warmup automation, email validation APIs, and detailed event tracking. SendGrid is best for technical teams who want fine-grained control over their sending infrastructure.

For more options, browse our best email deliverability tools and compare platforms in our tool reviews.

Advanced Deliverability Strategies

Once the fundamentals are solid, these strategies can further improve your inbox placement.

Custom Tracking Domains

Most email marketing tools rewrite links in your emails for click tracking. By default, these tracking links use the tool’s domain, not yours. Setting up a custom tracking domain (like track.yourdomain.com) improves deliverability because ISPs see your domain on the links rather than a shared tracking domain used by thousands of other senders.

Nearly every email platform supports custom tracking domains. Set this up during initial configuration.

Feedback Loops

Major ISPs offer feedback loops (FBLs) that notify you when a recipient marks your email as spam. Your email marketing tool should automatically process these and suppress complaining recipients. Verify this is working, because continuing to email people who have complained is one of the fastest paths to blocklisting.

BIMI (Brand Indicators for Message Identification)

BIMI lets you display your brand logo next to your emails in supporting email clients (currently Gmail and Apple Mail). It requires a verified DMARC policy at enforcement level (p=quarantine or p=reject) and a Verified Mark Certificate (VMC) for Gmail. While BIMI does not directly improve deliverability, it increases brand recognition and can improve open rates.

Dedicated Sending Domains

Use a subdomain (like mail.yourdomain.com) for marketing email rather than your primary domain. This isolates your marketing reputation from your main domain, protecting your corporate email if marketing deliverability issues arise. Most email tools support sending from subdomains and can guide you through the DNS setup.

Diagnosing Common Deliverability Problems

Emails Going to Gmail Spam

Gmail is the most common deliverability challenge. Check these in order:

1. Authentication: Verify SPF, DKIM, and DMARC pass for your domain in Google Postmaster Tools
2. Complaint rate: Must be below 0.1%. Even 0.15% can trigger spam filtering
3. Domain reputation: Check in Postmaster Tools. If it shows “Low” or “Bad,” you need to reduce volume, clean your list, and rebuild with highly engaged subscribers
4. Content: Send a test email to a Gmail account and check which tab it arrives in. If it is in Promotions, that is normal for marketing email. If it is in Spam, there is a content or reputation issue

Emails Going to Outlook/Hotmail Spam

Microsoft’s filters are particularly aggressive. Common solutions:

1. Check if your IP or domain is on Microsoft’s blocklist using their SNDS (Smart Network Data Services)
2. Reduce sending volume to Microsoft addresses temporarily
3. Focus on engaged Microsoft subscribers to rebuild reputation
4. Submit a sender support request through Microsoft’s deliverability support form

High Bounce Rates After Migration

When you switch email platforms, expect a temporary increase in bounces. The new platform sends from different IPs, and some receiving servers may be cautious initially. Warm up gradually and start with your most engaged subscribers.

Conclusion

Email deliverability is not a one-time setup task. It is an ongoing practice that combines proper technical configuration, consistent list hygiene, relevant content, and careful monitoring. Get the three pillars right: authentication, reputation, and content. Build from there.

The most important takeaway: send emails that people actually want to receive, to people who actually asked to receive them. This single principle solves most deliverability problems before they start.

If deliverability is your top concern, Postmark leads the category for transactional and reliability-focused use cases. Mailtrap is another strong option for teams that want email testing and sending in one platform, with a staging environment that catches deliverability issues before emails reach real inboxes. Elastic Email offers solid delivery infrastructure at a lower price point, starting at $19/month. For marketing email, MailerLite and ActiveCampaign both deliver strong inbox placement alongside their marketing features. See our best deliverability tools comparison and pricing breakdown to understand what each option costs.