Troubleshooting Email Delivery Issues

Overview

Email delivery issues — messages that bounce, arrive late, or land in spam — are among the most common problems domain owners face. This guide helps you systematically diagnose and resolve these issues.

Common Email Delivery Problems

1. Emails Bouncing Back

A bounce means the message was rejected by the receiving server. Bounces include an error code and reason:

• 550 User not found — The recipient address does not exist. Verify the spelling.
• 550 Sender rejected / blocked — Your sending IP or domain may be blacklisted. Check blacklist databases like MXToolbox Blacklist Check.
• 552 Mailbox full — The recipient’s inbox has reached its storage limit.
• 421 Too many connections — You are sending too many messages too quickly. Slow down your send rate.

2. Emails Going to Spam

If your messages consistently land in the spam folder of recipients:

• Check SPF — Ensure your domain has a valid SPF record that includes your sending server. Use nslookup -type=TXT yourdomain.com to verify.
• Check DKIM — Confirm that DKIM signing is enabled and the public key is published in DNS.
• Check DMARC — A DMARC policy helps receiving servers trust your messages.
• Review email content — Avoid spam trigger words, excessive links, large attachments, or image-only emails.
• Check your IP reputation — Use Google Postmaster Tools or similar services.

3. Emails Delayed

Delays can occur at multiple points:

• Greylisting — Some receiving servers temporarily reject messages from unknown senders. The sending server retries, causing a delay of several minutes.
• DNS propagation — If you recently changed MX records, some servers may still be routing to the old destination.
• Server overload — High load on either the sending or receiving server can queue messages.
• Large attachments — Very large emails take longer to transfer and may be throttled.

Diagnostic Steps

1. Read the bounce message — It almost always contains a specific error code and reason.
2. Verify MX records — Use nslookup -type=MX yourdomain.com or MXToolbox to confirm your MX records point to the correct mail server.
3. Test SPF, DKIM, DMARC — Send a test email to mail-tester.com for a comprehensive deliverability report.
4. Check blacklists — Verify your sending IP and domain are not listed on any major blacklists.
5. Review mail server logs — If you have access, server logs show exactly where delivery failed.
6. Check DNS propagation — Use whatsmydns.net to confirm your DNS records are visible globally.

Quick Fixes

When to Contact Support

If you have verified DNS records, checked blacklists, and reviewed bounce messages but the problem persists, contact 10Corp support with the full bounce message, the sender and recipient addresses, and the date and time the issue occurred.