How to Fix Email Bounce Back Error

An email bounce back (also called a Non-Delivery Report or NDR) means your message could not be delivered to the recipient. The bounce message usually contains an error code and a reason — but decoding it can be confusing.

This guide covers every common bounce type, what the SMTP error codes mean, and exactly how to fix each one.

Quick Fix Checklist

• ✅ Read the bounce error code (5xx = permanent, 4xx = temporary)
• ✅ Verify the recipient email address is spelled correctly
• ✅ Check your SPF, DKIM, and DMARC records
• ✅ Confirm your IP is not on any blacklists
• ✅ Verify your MX records are correct
• ✅ Check if recipient's mailbox is full
• ✅ Ensure your email is not too large (attachment limits)

Understanding Bounce Types

Hard Bounces (5xx errors)

Hard bounces are permanent failures. The email will never be delivered unless the underlying issue is fixed. Common causes:

• 550 — User not found: The email address doesn't exist
• 551 — User not local: The server doesn't handle mail for that domain
• 552 — Mailbox full: Recipient's storage quota exceeded
• 553 — Invalid address: Address format is malformed
• 554 — Transaction failed: General rejection (often spam filtering)

Soft Bounces (4xx errors)

Soft bounces are temporary failures. The server will retry delivery automatically:

• 421 — Service not available: Receiving server is temporarily down
• 450 — Mailbox busy: Try again later
• 451 — Server error: Receiving server encountered an error
• 452 — Insufficient storage: Server storage is temporarily full

Step-by-Step Fix

Step 1: Read the Bounce Message

Every bounce email contains diagnostic information. Look for:

# Key information in bounce messages:
# 1. SMTP error code (e.g., 550 5.1.1)
# 2. Remote server response
# 3. Recipient address
# 4. Timestamp

# Example bounce header:
# Remote-MTA: dns; gmail-smtp-in.l.google.com
# Diagnostic-Code: smtp; 550-5.1.1 The email account does not exist

Step 2: Fix "User Not Found" (550 5.1.1)

This is the most common bounce. The recipient address doesn't exist:

• Double-check spelling of the email address
• Confirm the recipient hasn't changed their email
• If it's your own domain, verify the mailbox exists in your control panel

Step 3: Fix "Rejected by Policy" (550 5.7.1)

Your email was rejected by the recipient's spam filter:

# Check your authentication records
dig TXT yourdomain.com +short | grep spf
dig TXT default._domainkey.yourdomain.com +short
dig TXT _dmarc.yourdomain.com +short

# Check blacklists
# Visit: https://mxtoolbox.com/blacklists.aspx

Fix: Ensure SPF, DKIM, and DMARC are properly configured.

Step 4: Fix "Mailbox Full" (552)

The recipient's mailbox has exceeded its storage quota. You cannot fix this — the recipient needs to clear space. Contact them through another channel.

Step 5: Fix "Message Too Large" (552 5.3.4)

Your email (with attachments) exceeds the recipient server's size limit:

• Most servers limit messages to 25-50 MB
• Compress attachments or use a file-sharing link instead
• Remove unnecessary inline images

Step 6: Fix DNS-Related Bounces

# Verify MX records for recipient domain
dig MX recipientdomain.com +short

# Verify your own MX records
dig MX yourdomain.com +short

# Check if DNS resolves properly from your server
nslookup recipientdomain.com

Check Exim Logs (cPanel Servers)

# View recent bounces
grep "rejected\|bounced\|failed" /var/log/exim_mainlog | tail -30

# Check specific recipient
grep "[email protected]" /var/log/exim_mainlog | tail -10

# View frozen messages (failed delivery)
exim -bp | grep frozen

Common Mistakes

• Ignoring bounce codes: The error code tells you exactly what's wrong — always read it first
• Retrying hard bounces: 5xx errors won't resolve themselves. Fix the root cause before resending.
• No SPF/DKIM: Missing authentication causes policy rejections at Gmail, Yahoo, and Outlook
• Shared IP reputation: On shared hosting, other users' spam can get your IP blacklisted
• Sending to old lists: Purchased or stale email lists contain many invalid addresses

Frequently Asked Questions

What does "550 5.1.1 User Unknown" mean?

It means the email address you're sending to doesn't exist on the recipient's mail server. Double-check the spelling, or contact the person through another channel to confirm their current email address.

Why am I getting bounces only to Gmail?

Gmail has strict sender requirements since February 2024. You must have valid SPF, DKIM, and DMARC records, a clean IP reputation, and proper PTR records. Check our guide on fixing emails going to spam.

How do I stop getting bounce notifications for spam?

If you're receiving bounces for emails you didn't send, your domain may be spoofed. Implement a strict DMARC policy (p=reject) to prevent unauthorized senders from using your domain.

Can I resend an email after a soft bounce?

Yes, soft bounces (4xx) are temporary. Your mail server will automatically retry delivery. If it persists after 24-48 hours, investigate the specific error code.