Troubleshooting Email Deliverability Problems

Email deliverability problems almost always fall into one of four categories: authentication failures, reputation damage (blacklisting), content/list issues, or technical misconfigurations. Work through this checklist systematically.

Step 1: Identify the Symptom

Different symptoms point to different causes:

Step 2: Check Email Authentication

Run a DomainScan email authentication check on your sending domain.

SPF — must show:

• Record exists at root domain
• All your sending services included
• No more than 10 DNS lookups
• -all or ~all qualifier at the end

DKIM — must show:

• At least one valid key for your primary sending service
• Key is 2048-bit RSA or Ed25519 (not 1024-bit)
• Not in test mode (t=y)

DMARC — must show:

• Record exists at _dmarc.yourdomain.com
• Policy is p=quarantine or p=reject (not p=none)
• rua=mailto: set for aggregate reports

Fix any failures here before proceeding. Authentication is the foundation — everything else sits on top.

Step 3: Check Blacklists

Check both your domain and your mail server’s IP:

1. Enter your domain at DomainScan /domain/blacklist
2. Also check your outbound SMTP server’s IP (find it via dig MX yourdomain.com → then dig A that-hostname)

If listed:

• Identify the list and the listing reason
• Fix the root cause (spam complaints, compromised account, malware)
• Submit removal request to the specific list’s delist form
• Monitor for re-listing over the following 7 days

Step 4: Check Google Postmaster Tools

For Gmail deliverability specifically, Postmaster Tools is authoritative:

1. Go to postmaster.google.com
2. Verify your domain
3. Check Domain Reputation (should be High)
4. Check Spam Rate (should be under 0.1%; Google flags at 0.3%)
5. Check Authentication tab — all should show Pass

A domain reputation below “High” means Gmail applies extra spam filtering to all your email, regardless of content.

Step 5: Check Microsoft SNDS

For Outlook/Hotmail deliverability:

1. Go to sendersupport.olc.protection.outlook.com
2. Register and verify your sending IP
3. Check the traffic light status (green/yellow/red)
4. Red means your IP is being blocked — submit a delist request

Step 6: Verify MX Records and Mail Server Configuration

Sometimes the problem is receiving, not sending. Check:

# Verify MX records resolve correctly
dig MX yourdomain.com
dig A <mx-hostname>

# Test SMTP connection to your mail server
telnet mail.yourdomain.com 25

Common server misconfigurations that hurt deliverability:

• No PTR (reverse DNS): Mail server IP has no PTR record. Many servers reject or penalize this. Ask your hosting provider to set PTR → mail.yourdomain.com.
• Missing STARTTLS: Server doesn’t offer TLS for outbound connections. Many providers now require TLS.
• Wrong HELO/EHLO: Server introduces itself with a hostname that doesn’t match its PTR record.

Step 7: Analyze Bounce Messages

Bounce messages contain delivery status codes. Read them carefully:

550 5.7.1 [IP] listed at zen.spamhaus.org     → blacklisted
550 5.7.26 This message does not pass auth     → DMARC failure
421 Too many connections from your IP          → rate limited
550 5.1.1 User unknown                         → invalid recipient address
452 4.2.2 Mailbox full                         → soft bounce, try later

5xx codes = permanent failures (remove from list) 4xx codes = temporary failures (retry later, suppress after repeated failures)

Step 8: Review List Quality

Bad list hygiene causes deliverability problems that no authentication fix can solve:

• Hard bounce rate above 2%: Sending to too many invalid addresses → clean your list
• Unsubscribe rate above 0.5%: Content isn’t matching expectations → improve targeting
• Spam complaint rate above 0.08%: People don’t remember signing up → use confirmed opt-in
• Low open rates (under 10%): Sending to disengaged subscribers hurts reputation → sunset inactive contacts after 90 days

Step 9: Check Email Content

Content filters look for:

• Spam trigger words in subject line (“FREE”, “URGENT”, “Act now”, excessive caps)
• Too many images, no plain text version
• Links to blacklisted domains
• Mismatched From: address and sending domain
• Missing unsubscribe link (required by CAN-SPAM and GDPR)
• URL shorteners hiding the real destination

Test your email content at mail-tester.com (free, checks content + authentication + blacklists).

Step 10: Check Sending Infrastructure

If you’re self-hosting email:

# Verify your server's PTR record
dig -x <your-outbound-IP>

# Check SMTP banner matches hostname
openssl s_client -connect mail.yourdomain.com:587 -starttls smtp

# Verify your server isn't an open relay (dangerous)
telnet mail.yourdomain.com 25
EHLO test.com
MAIL FROM:<test@test.com>
RCPT TO:<external@gmail.com>
# Should respond: 554 Relay access denied

Escalation: Email Provider Support

If you’ve completed all steps and deliverability is still broken:

• Gmail: Use Postmaster Tools → Feedback → submit a review request
• Microsoft 365 / Outlook: sendersupport.olc.protection.outlook.com → Delist Portal
• Yahoo/AOL: senders.yahooinc.com → report a deliverability problem
• Your own ESPEmail Service Provider (SendGrid, Mailgun, etc.): contact their deliverability team — they have more leverage with major ISPs than you do individually