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.

Authentication: Foundation of Deliverability

SPF, DKIM, and DMARC

These are the most critical factors.

SPF (Sender Policy Framework):

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

Result: Receiving servers verify your email comes from authorized IPs
Benefit: Dramatically improves deliverability
Impact: Without SPF, many emails marked spam or rejected

DKIM (DomainKeys Identified Mail):

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

Result: Cryptographic signature proves authenticity
Benefit: Gmail, Yahoo, Hotmail heavily weight DKIM
Impact: Missing DKIM = lower inbox placement

DMARC (Domain-based Message Authentication):

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

Result: Enforces SPF/DKIM alignment
Benefit: Improves reputation, enables BIMI
Impact: Essential for ESP customers

Implementation checklist:

SPF record includes all 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 sending IP:

Build reputation:

Start with low volume (warm-up)
Gradually increase volume over weeks
Maintain consistent sending patterns
Avoid sudden spikes (flagged as botnet)

Warm-up schedule example:

Week 1: 100 emails/day
Week 2: 500 emails/day
Week 3: 1,000 emails/day
Week 4: 5,000 emails/day
Gradually reach target volume

Protect reputation:

Remove bounces immediately
Monitor feedback loops
Don't email inactive subscribers
Maintain list hygiene
Respect unsubscribe requests

Domain Reputation

Receiving servers evaluate sending domain:

Build domain reputation:

Consistent, authentic content
No spam complaints
Active subscriber engagement
Authentication (SPF/DKIM/DMARC)
Real, verifiable business

Monitor reputation:

# Check domain blacklist status
# Tools: MXToolbox, Google Postmaster Tools
# Look for: Domain Rep score, complaints

# Monitor feedback loops
# Set up FBL for major ISPs
# ISP notifies you of spam complaints
# Remove those subscribers immediately

Email List Quality

Subscriber Acquisition

Quality over quantity:

Good practices:

Explicit opt-in (not bought lists)
Clear description of what they're signing up for
Clean capture forms
Verify email addresses
Remove invalid addresses immediately

Bad practices:

Buying email lists
Scraping addresses from web
Sending to random addresses
Not verifying addresses
No opt-in consent

Impact: Dirty lists cause spam complaints and blacklisting

List Hygiene

Regular cleaning prevents deliverability issues:

# Bounce handling
- Hard bounces: Remove immediately
- Soft bounces: Remove after 3-5 bounces
- Never retry hard bounces

# Spam traps
- Old abandoned addresses monitored by ISPs
- Sending to these causes blacklisting
- Monitor list source, remove suspicious addresses

# Engagement scoring
- Track opens and clicks
- Remove non-engaged subscribers after 6-12 months
- Periodic re-engagement campaigns

# Validation services
- Real-time validation on signup
- Batch validation of existing lists
- Detects spam traps, syntax errors

Removing Unengaged Subscribers

Non-engaged subscribers hurt deliverability:

Engagement metrics:

Last open: >6 months ago?
Last click: >12 months ago?
Never opened or clicked anything?

Win-back campaign:

Send to inactive subscribers:
"We miss you! Haven't heard from you in a while."
"Confirm you want to stay subscribed"

Results:
- Some re-engage
- Others confirm they want to unsubscribe
- Remove those who don't respond (30 days)

Benefit: Removes non-engaged, improves open rates

Content Quality

Subject Line and Preview

Avoid spam trigger words:

Bad (likely spam folder):
- "FREE!!! Limited time!!!!"
- "You won! Claim prize now!"
- "Click here immediately"

Good (natural language):
- "Update on your account"
- "New resources available"
- "Your monthly summary"

Preview text matters:

Preview shows first line of email
Make it valuable, not clickbait
Helps ISPs assess legitimacy

Email Content

Content practices:

Proper HTML formatting (not just images)
Plain text alternative
Balance images to text (1:1 ratio)
Real addresses and unsubscribe links
Authentication headers
No suspicious links or attachments

Avoid:

Exaggeration and ALL CAPS
Too many exclamation points
Multiple fonts and colors
Heavy image-only emails
Suspicious links
Attachments (use links instead)

Personalization

Personalized emails get better engagement:

Bad: "To our valued customer"
Good: "Hi John, here's what we found..."

Bad: Generic content
Good: Personalized based on behavior

Bad: Blast to everyone
Good: Segmented by interest/behavior

Benefits:

Higher open and click rates
Lower spam complaints
Better sender reputation
ISPs note engagement

Technical Configuration

Dedicated IP vs. Shared IP

Shared IP (free ESPs):

Easy to start
Reputation tied to other senders
One bad sender hurts all
Good for small volume

Dedicated IP (premium ESPs):

Build own reputation
Not affected by others
Requires reputation building
Good for medium-large senders

Return Path Configuration

Return Path (bounce domain):

MAIL FROM: [email protected]
RCPT TO: [email protected]

Should be:
- Same domain as From address
- Owned by your company
- DKIM signed
- Monitored for bounces

Misconfiguration causes deliverability issues:

From: [email protected]
Return-Path: [email protected]
← Mismatched = spam risk

IP Warming

New IPs need reputation building:

Warm-up process:

Day 1-2: Test send to yourself (small volume)
Day 3-4: Send to engaged subscribers (100s)
Day 5-6: Increase to 1000s
Day 7-10: Ramp to 10,000s
Week 2-3: Increase to target volume
Week 4+: Stable sending at full volume

Don't:

Don't blast large list immediately
Don't send to inactive subscribers during warmup
Don't suddenly stop (looks suspicious)
Don't violate ISP rate limits

ISP-Specific Considerations

Gmail (largest provider)

Gmail priorities:

Authentication (SPF/DKIM/DMARC) - critical
Sender reputation
List quality (engagement)
Content quality
BIMI (optional but helpful)

Gmail Postmaster Tools:

Free account monitoring
Shows reputation metrics
Identifies issues
Highly recommended

Yahoo/AOL

Similar to Gmail:

Authentication critical
Reputation important
List quality matters
FBL (feedback loop) available

Microsoft (Outlook.com, etc.)

Microsoft requirements:

SPF/DKIM/DMARC needed
IP reputation matters
Complaint rate crucial
SNDS (Sender Notification Data Service) shows issues

Monitoring and Troubleshooting

Key Metrics

Track these:

Delivery rate: % of emails delivered (vs. bounced)
Target: >98%

Open rate: % of delivered emails opened
Good: 15-25% (industry varies)
Bad: <5%

Click rate: % of opened emails with clicks
Good: 2-5%
Bad: <1%

Complaint rate: % marked as spam
Target: <0.1%
Bad: >1%

Bounce rate: % undeliverable
Target: <3%
Bad: >5%

Tools for Monitoring

Google Postmaster Tools:

Free for Gmail senders
Shows reputation, deliverability issues
Heat maps of engagement
Highly recommended

Sender Score (ReturnPath):

Free IP reputation checker
0-100 scale
Shows which ISPs have issues

250ok, Dyspatch:

Premium monitoring
Detailed diagnostics
ISP-specific feedback

Mail Tester:

Test single emails
Check content for issues
Spam filter scoring
Free option available

Common Issues and Fixes

Issue: Low open rate

Possible causes:
- Subject line not engaging
- Sending to inactive subscribers
- Content not relevant
- Poor sender reputation

Fixes:
- A/B test subject lines
- Segment list by engagement
- Improve content
- Check reputation metrics

Issue: High bounce rate

Possible causes:
- Purchased list
- Old list, addresses changed
- Typos in addresses
- Invalid addresses

Fixes:
- Use list validation service
- Remove old addresses
- Improve signup validation
- Switch list source

Issue: Spam folder placement

Possible causes:
- Missing authentication
- Poor sender reputation
- Content triggers spam filters
- Complaint rate too high

Fixes:
- Implement SPF/DKIM/DMARC
- Warm up IP
- Improve content
- Remove complainers

Issue: ISP-specific problems

Gmail low: Check Gmail Postmaster Tools
Outlook low: Check complaint rate, IP reputation
Yahoo low: Check FBL, authentication

Solutions:
- Follow ISP-specific guidance
- Increase engagement
- Improve authentication
- Segment by ISP if needed

Compliance Matters Too

CAN-SPAM (United States)

Requirements:

Clear From address
Honest subject line
Physical mailing address
Easy unsubscribe
Honor unsubscribe within 10 days

Violations = FTC enforcement, fines

GDPR (European Union)