How to improve email deliverability?

Improving Email Deliverability: Complete Guide

Email deliverability—the ability to get emails into recipients' inboxes rather than spam folders—depends on technical infrastructure, sender reputation, and content quality. Many organizations see 10-20% of legitimate emails end up in spam. Improving deliverability requires addressing multiple factors systematically, starting with authentication and extending through content practices, list hygiene, and ongoing monitoring.

Authentication: Foundation of Deliverability

SPF, DKIM, and DMARC

Authentication records are the most critical factors determining whether your emails reach inboxes. Without proper authentication, major email providers like Gmail, Yahoo, and Outlook will either mark your messages as spam or reject them entirely.

SPF (Sender Policy Framework) tells receiving servers which IP addresses are authorized to send email on behalf of your domain. When a server receives email claiming to be from your domain, it checks the SPF record to verify the sending IP is authorized. Without SPF, many emails are marked as spam or rejected outright because the receiver cannot verify authenticity.

example.com TXT: v=spf1 include:sendgrid.net include:mailchimp.com ~all

DKIM (DomainKeys Identified Mail) adds a cryptographic signature to your emails proving they haven't been modified in transit and genuinely originated from your domain. Gmail, Yahoo, and Hotmail heavily weight DKIM presence when making inbox placement decisions. Missing DKIM signatures correlate strongly with lower inbox placement rates.

selector._domainkey.example.com: v=DKIM1; k=rsa; p=[KEY]

DMARC (Domain-based Message Authentication) enforces alignment between SPF/DKIM and the visible "From" address, preventing attackers from spoofing your domain. DMARC also enables BIMI (Brand Indicators for Message Identification), which displays your logo in recipient inboxes. For organizations using email service providers, DMARC is essential for maintaining deliverability.

_dmarc.example.com: v=DMARC1; p=reject; rua=mailto:[email protected]

Implementation checklist:

SPF record includes all sending services
DKIM signing configured on mail server
DKIM keys rotated within 1-2 years
DMARC policy set to p=reject (not p=none)
Monitoring DMARC aggregate reports
All subdomains protected

Sender Reputation

IP Reputation

Receiving servers evaluate the reputation of your sending IP address based on historical behavior. Building positive IP reputation requires starting with low email volume and gradually increasing over several weeks while maintaining consistent sending patterns. Sudden volume spikes trigger spam filters designed to catch botnets, so patience during the warmup process is essential.

A typical IP warmup schedule starts with 100 emails per day in week one, increases to 500 in week two, 1,000 in week three, and 5,000 in week four before gradually reaching target volume. Throughout this process, you must protect your reputation by removing bounced addresses immediately, monitoring feedback loops from major ISPs, avoiding emails to inactive subscribers, maintaining rigorous list hygiene, and honoring unsubscribe requests promptly.

Domain Reputation

Beyond IP reputation, receiving servers also evaluate your sending domain's history. Domain reputation builds through consistent, authentic content that generates engagement rather than complaints. Organizations with strong domain reputation have active subscribers who open and click emails, authentication properly configured, and verifiable business information.

Monitor domain reputation using tools like MXToolbox and Google Postmaster Tools. Set up feedback loops (FBLs) with major ISPs so you're notified when recipients mark your email as spam, then remove those subscribers immediately to prevent further complaints.

Email List Quality

Subscriber Acquisition

Quality matters far more than quantity when building email lists. Lists built through explicit opt-in where subscribers clearly understand what they're signing up for generate dramatically better deliverability than purchased lists or scraped addresses. Clean capture forms that verify email addresses and remove invalid entries immediately prevent deliverability problems before they start.

Buying email lists, scraping addresses from websites, sending to random addresses, skipping address verification, and collecting addresses without clear opt-in consent all lead to spam complaints and blacklisting that can take months to recover from.

List Hygiene

Regular list cleaning prevents the deliverability issues that accumulate when stale data remains in your system. Handle bounces promptly by removing hard bounces immediately and soft bounces after three to five failed attempts. Never retry hard bounced addresses, as doing so damages your reputation.

Spam traps present particular danger because they're old abandoned addresses that ISPs now monitor specifically to identify senders using dirty lists. Sending to spam traps causes immediate blacklisting. Monitor list sources carefully and remove any addresses that seem suspicious.

Track engagement by measuring opens and clicks for each subscriber. Remove non-engaged subscribers after six to twelve months of inactivity, or run periodic re-engagement campaigns to identify who wants to remain subscribed. Use validation services both for real-time validation during signup and batch validation of existing lists to detect spam traps and syntax errors.

Removing Unengaged Subscribers

Non-engaged subscribers actively hurt your deliverability because low engagement signals to ISPs that recipients don't value your emails. Evaluate engagement by identifying subscribers who haven't opened an email in more than six months, haven't clicked anything in more than twelve months, or have never opened or clicked anything despite receiving multiple emails.

Before removing unengaged subscribers, run a win-back campaign asking them to confirm they want to remain subscribed. This message might say something like "We miss you! It's been a while since you opened our emails. Click here to confirm you want to stay subscribed." Some recipients will re-engage, others will explicitly unsubscribe, and those who don't respond within 30 days should be removed. This process improves open rates by removing subscribers who drag down your metrics.

Content Quality

Subject Line and Preview

Subject lines significantly influence both whether recipients open your email and whether spam filters flag it. Certain patterns trigger spam filters consistently, including excessive punctuation like "FREE!!! Limited time!!!!", prize claims like "You won! Claim prize now!", and urgent imperatives like "Click here immediately."

Natural language subject lines perform better with both recipients and filters. Examples include "Update on your account," "New resources available," and "Your monthly summary." Preview text also matters because it shows the first line of your email in inbox listings. Make preview text valuable rather than clickbait, as ISPs assess legitimacy based on this content.

Email Content

Well-structured content improves deliverability alongside engagement. Use proper HTML formatting with a balance of text and images rather than image-only emails, include a plain text alternative for recipients whose clients don't render HTML, maintain approximately equal proportions of images to text, include real physical addresses and working unsubscribe links, and ensure authentication headers are present.

Content to avoid includes exaggeration, ALL CAPS text, excessive exclamation points, multiple fonts and colors that look unprofessional, heavy image-only emails that spam filters flag as potentially hiding content, suspicious links especially URL shorteners, and attachments which you should replace with download links.

Personalization

Personalized emails generate better engagement, which in turn improves sender reputation. Rather than generic salutations like "To our valued customer," address recipients by name with specific content like "Hi John, here's what we found based on your recent activity." Rather than blasting identical content to everyone, segment by interest and behavior to ensure relevance.

Personalization benefits compound over time. Higher open and click rates lead to lower spam complaints, which builds better sender reputation, which ISPs notice and reward with better inbox placement.

Technical Configuration

Dedicated IP vs. Shared IP

Shared IPs used by free email service providers make starting easy but tie your reputation to other senders on the same IP. One bad sender can hurt everyone sharing that IP. Shared IPs work well for small-volume senders who can't justify building their own reputation.

Dedicated IPs available through premium email service providers let you build your own reputation independent of other senders. You're not affected by others' behavior but must invest time in IP warmup. Dedicated IPs suit medium to large senders who send consistently enough to maintain reputation.

Return Path Configuration

The Return Path (also called bounce domain) should match your From address domain and be owned by your company. Mismatches between the From address and Return Path raise spam flags. For example, if your From address is [email protected] but your Return Path is [email protected], this mismatch increases spam risk. Ensure your Return Path domain is DKIM signed and monitored for bounces.

IP Warming

New IP addresses have no reputation, which ISPs treat with suspicion. The warmup process builds trust gradually. Start with test sends to yourself for the first day or two, then send to your most engaged subscribers in volumes of hundreds. Increase to thousands by day five or six, then tens of thousands by day seven to ten. During weeks two and three increase toward target volume, reaching stable full-volume sending by week four or later.

During warmup, avoid blasting large lists immediately, sending to inactive subscribers, suddenly stopping which looks suspicious, and violating ISP rate limits. Patience during warmup prevents deliverability problems that take far longer to fix than the warmup period itself.

ISP-Specific Considerations

Gmail

As the largest email provider, Gmail's requirements effectively set industry standards. Gmail prioritizes authentication first (SPF/DKIM/DMARC is critical), then sender reputation, list quality measured through engagement, content quality, and optionally BIMI which helps but isn't required. Google Postmaster Tools provides free monitoring showing reputation metrics and identifying issues. Every organization sending email should register.

Yahoo and AOL

Yahoo and AOL follow similar priorities to Gmail: authentication is critical, reputation important, and list quality matters. Both offer feedback loops you should subscribe to for spam complaint notifications.

Microsoft (Outlook.com)

Microsoft requires SPF/DKIM/DMARC, weighs IP reputation heavily, and pays particular attention to complaint rates. Microsoft's SNDS (Sender Notification Data Service) shows delivery issues affecting Outlook recipients.

Monitoring and Troubleshooting

Key Metrics

Track these metrics continuously to catch problems early:

Metric	Target	Warning Level
Delivery rate	>98%	<95%
Open rate	15-25%	<5%
Click rate	2-5%	<1%
Complaint rate	<0.1%	>1%
Bounce rate	<3%	>5%

Tools for Monitoring

Google Postmaster Tools provides free Gmail delivery analytics showing reputation, deliverability issues, and engagement heat maps. Sender Score from ReturnPath offers free IP reputation checking on a 0-100 scale showing which ISPs have issues with your IP. Premium services like 250ok and Dyspatch provide detailed diagnostics and ISP-specific feedback. Mail Tester lets you test individual emails for content issues and spam filter scoring.

Common Issues and Fixes

Low open rates typically result from unengaging subject lines, sending to inactive subscribers, irrelevant content, or poor sender reputation. Fix these by A/B testing subject lines, segmenting lists by engagement, improving content relevance, and checking reputation metrics.

High bounce rates usually indicate purchased lists, old lists with changed addresses, typos in signup forms, or invalid addresses from poor validation. Address these by using list validation services, removing old addresses, improving signup form validation, and evaluating list sources.

Spam folder placement stems from missing authentication, poor sender reputation, content that triggers spam filters, or complaint rates that are too high. Solutions include implementing SPF/DKIM/DMARC, warming up IPs properly, improving content, and removing subscribers who complain.

ISP-specific problems require targeted investigation. If Gmail deliverability is low, check Google Postmaster Tools. For Outlook problems, investigate complaint rates and IP reputation. Yahoo issues usually relate to feedback loop complaints or authentication gaps.

Compliance Matters Too

CAN-SPAM (United States)

CAN-SPAM requires clear From addresses that accurately identify the sender, honest subject lines that don't mislead recipients, physical mailing addresses in every commercial email, easy unsubscribe mechanisms, and honoring unsubscribe requests within 10 business days. Violations result in FTC enforcement and substantial fines.

GDPR requires explicit opt-in consent before sending marketing emails (double opt-in is recommended), easy unsubscribe mechanisms, clear privacy policies explaining how data is used, appropriate data security measures, and responsiveness to data access and deletion requests. Violations can result in fines up to €20 million or 4% of global annual revenue.

Other Regulations

CASL governs email in Canada, PIPEDA addresses Canadian data privacy, PDPA applies in Singapore, and numerous other country-specific laws may apply depending on where your recipients are located. Compliance with these regulations improves deliverability because it requires the same practices that build sender reputation: clear opt-in, easy unsubscribe, quality lists, and reducing complaints.

Complete Deliverability Checklist

Authentication

List Quality

Content

Infrastructure

Monitoring

Success Metrics

Target these outcomes to confirm deliverability improvements are working:

Metric	Target
Delivery rate	>98%
Open rate	15-25%
Click rate	2-5%
Complaint rate	<0.1%
Bounce rate	<3%
List growth	Increasing
Engagement	Improving

Conclusion

Email deliverability requires attention to technical configuration including authentication and infrastructure, reputation management through IP warming and consistent sending, list quality through hygiene and engagement monitoring, content that avoids spam triggers while providing value, and ongoing monitoring to catch and address issues quickly.

Organizations that implement these practices consistently achieve 95%+ inbox placement and strong engagement. Poor deliverability is almost always solvable through systematic attention to these factors. Start with authentication and list quality—these drive the largest improvements. Monitor metrics continuously, respond to issues quickly, and refine based on data.