Improving Email Deliverability

What is Email Deliverability?

Email deliverability is the ability of your emails to reach recipients' inboxes rather than being filtered to spam, bounced, or silently dropped. It is distinct from email delivery, which simply means the message was accepted by the receiving server. A message can be "delivered" but still end up in the spam folder.

Deliverability is influenced by three broad categories: your sender reputation, the content and structure of your messages, and how recipients engage with your email. Improving deliverability requires attention to all three.

The Deliverability Equation

Think of deliverability as a formula with three core components:

Reputation

Your sender reputation is the single largest factor in deliverability. It encompasses your domain reputation, IP reputation, authentication status, blacklist status, and historical sending behavior. For a deep dive, see our guide on What is Sender Reputation?

Content

While content-based spam filtering is less dominant than it was a decade ago, the structure, formatting, and language of your emails still matter. Mailbox providers analyze subject lines, HTML structure, image-to-text ratio, link density, and the overall "spamminess" of your content.

Engagement

How recipients interact with your emails sends powerful signals to mailbox providers. High open rates, click rates, replies, and forwards indicate that your email is wanted. Low engagement, high delete-without-reading rates, and spam complaints indicate the opposite.

Authentication Foundation

Before addressing any other deliverability factor, ensure your email authentication is properly configured. This means having valid SPF, DKIM, and DMARC records for every domain you send from.

Authentication is the minimum requirement for credibility. Without it, mailbox providers have no way to verify that your messages are legitimate. In 2024, Google and Yahoo made authentication mandatory for all bulk senders.

Quick links to verify your authentication setup:

• Check your SPF record
• Check your DKIM record
• Check your DMARC record

For a full walkthrough of setting up authentication, see our Email Authentication Guide.

List Hygiene Best Practices

The quality of your email list directly impacts every aspect of deliverability. A clean list means fewer bounces, fewer complaints, and better engagement metrics.

Use Double Opt-In

Double opt-in (also called confirmed opt-in) requires new subscribers to click a confirmation link in an email before being added to your list. This ensures that the email address is valid, that the person actually owns it, and that they genuinely want to receive your emails.

While double opt-in may reduce the total number of signups compared to single opt-in, it dramatically improves list quality. Double opt-in lists consistently produce higher engagement rates, lower bounce rates, and fewer spam complaints.

Remove Hard Bounces Immediately

When an email address hard bounces (permanently undeliverable), remove it from your list after the first occurrence. Continuing to send to invalid addresses wastes resources and damages your reputation. Most ESPs handle this automatically, but verify that your bounce handling is properly configured.

Prune Inactive Subscribers

Subscribers who have not opened or clicked any of your emails in 6-12 months are dragging down your engagement metrics. Before removing them, try a re-engagement campaign: send one or two emails asking if they still want to hear from you. If they do not respond, remove them from your active list.

Validate Addresses at Signup

Implement basic email validation at the point of signup. Check for proper email format, valid domain MX records, and common typos (like "gmial.com" instead of "gmail.com"). This prevents invalid addresses from entering your list in the first place.

Monitor for Role-Based Addresses

Role-based email addresses (info@, admin@, sales@, support@) often forward to multiple people, are associated with higher complaint rates, and may be monitored by anti-spam systems. Consider excluding them from marketing campaigns.

Content Optimization

While reputation is the primary deliverability factor, content quality and structure still play a role. Here are the key content practices to follow:

Subject Lines

• Write clear, honest subject lines that accurately describe the content
• Avoid ALL CAPS, excessive punctuation (!!!), and misleading language
• Do not use "Re:" or "Fwd:" unless the email is actually a reply or forward
• Avoid spam trigger words like "FREE!!!", "Act Now", "Limited Time", "Guaranteed"
• Keep subject lines concise, ideally under 50 characters

HTML Structure

• Use clean, table-based HTML for maximum compatibility across email clients
• Maintain a healthy balance of text and images; do not send image-only emails
• Include a plain-text version of every HTML email (multipart/alternative)
• Keep your HTML file size under 100KB
• Use inline CSS rather than external stylesheets

Links

• Always use full URLs (not URL shorteners like bit.ly, which are commonly used by spammers)
• Do not include too many links in a single email; focus on a clear call to action
• Ensure all links point to legitimate, accessible web pages
• Never link to domains that are blacklisted or have a poor reputation

Unsubscribe Handling

• Include a visible unsubscribe link in every marketing email
• Support the List-Unsubscribe header (both mailto and URL methods)
• Support one-click unsubscribe (required by Google and Yahoo for bulk senders)
• Honor unsubscribe requests within 2 business days (preferably immediately)

Sending Patterns and Volume Management

Warm Up New IPs and Domains

When you start sending from a new IP address or domain, begin with a small volume of emails sent to your most engaged recipients. Gradually increase volume over 2-4 weeks. A typical warmup schedule might look like this:

Maintain Consistent Volume

Mailbox providers look for consistent sending patterns. Sending 10,000 emails every day is better for your reputation than sending nothing for a week and then blasting 70,000 at once. If your sending volume varies naturally (for example, e-commerce with seasonal peaks), ramp up gradually before high-volume periods.

Time Your Sends Thoughtfully

While the best send time depends on your specific audience, avoid sending large volumes in the middle of the night (recipient's local time) or on weekends when engagement tends to be lower. Lower engagement rates send negative signals to mailbox providers.

Segment Your Audience

Send targeted, relevant content to different segments of your list rather than the same message to everyone. Segmented campaigns produce higher engagement, which boosts your sender reputation. Common segmentation approaches include:

• Activity level (highly engaged, moderately engaged, inactive)
• Purchase history or product interest
• Geographic location and time zone
• Subscription date (new vs long-term subscribers)

Monitoring and Continuous Improvement

Deliverability is not a one-time project. It requires ongoing monitoring and adjustment. Here are the key metrics and tools to watch:

Key Metrics to Track

• Bounce rate - Keep hard bounces below 2%. See our Bounce Rate Guide.
• Complaint rate - Keep below 0.1% (1 per 1,000). Google warns at 0.3%.
• Open rate - Varies by industry, but declining open rates may signal deliverability problems.
• Inbox placement rate - The percentage of sent emails that reach the inbox (not spam).
• Blacklist status - Check regularly with our Blacklist Checker.

Tools for Monitoring

• Sender Reputation Checker - Get your Sender Reputation Score
• Google Postmaster Tools - Domain and IP reputation at Gmail
• Microsoft SNDS - Sending reputation at Outlook/Hotmail
• Your ESP's built-in analytics and deliverability dashboard

Set Up Alerts

Configure alerts for unusual changes in your key metrics. A sudden spike in bounces or complaints may indicate a list quality issue, a server compromise, or a content problem. Early detection lets you address issues before they significantly impact your reputation.

Deliverability Checklist

Use this checklist as a reference for maintaining healthy email deliverability: