Emails Bouncing Back

When an email bounces, you receive a delivery failure notification (sometimes called a Non-Delivery Report or NDR) explaining why the message couldn't be delivered. Understanding these bounce messages is the first step to fixing the problem. This guide covers the most common bounce reasons and what to do about each one.

01. Reading Bounce Messages

Every bounce message contains three key pieces of information:

The SMTP status code - A three-digit code like 550, 421, or 552. Codes starting with 4xx are temporary (the server might accept the message later). Codes starting with 5xx are permanent (the message will not be delivered unless you fix something).

The enhanced status code - A three-part code like 5.1.1 or 4.7.0 that gives more detail. The first number matches the SMTP code (4 = temporary, 5 = permanent), and the remaining numbers categorize the error type.

The diagnostic text - A plain-language explanation from the receiving server, like "Mailbox not found" or "Message rejected due to spam content."

02. Common Bounce Error Codes

550 5.1.1 - User Unknown / Mailbox Not Found

The email address you sent to doesn't exist on the recipient's mail server. Check for typos in the address. If the address was valid previously, the recipient may have deleted their account or the domain's MX records may have changed.

550 5.2.1 - Mailbox Full / Over Quota

The recipient's mailbox has reached its storage limit. They need to delete messages or increase their quota. You can try again later after they've freed up space.

552 5.3.4 - Message Too Large

Your email (including attachments) exceeds the recipient server's maximum message size. Reduce attachment size by compressing files or using a file sharing service instead. Ultra Web Hosting's outbound limit is 50MB per message, but many recipient servers cap at 25MB or less.

421 4.7.0 - Temporary Rate Limit / Try Again Later

The recipient server is temporarily refusing connections, usually because too many messages were sent in a short period. This is a temporary error - the server will retry automatically. If you see this frequently, you may be sending too fast.

550 5.7.1 - Message Rejected / Blocked by Policy

The receiving server rejected the message based on content, sender reputation, or policy. This is the most common bounce for spam-related rejections. See the blacklist and authentication sections below.

550 5.7.25 - PTR Record Not Found

The recipient server checked the sending IP's reverse DNS (PTR record) and found it missing or mismatched. This is configured at the server level - contact support if you see this error.

03. IP Blacklisting

If the bounce message mentions a blacklist (DNSBL, Spamhaus, Barracuda, etc.), the mail server's IP address has been listed on a reputation blacklist. This can happen because of spam originating from another account on the same shared server, a compromised email account, or a compromised WordPress site sending spam.

To check if the server IP is blacklisted, use our DNS Lookup and Blacklist Check tool. If you find a listing, open a support ticket and we'll work on getting the IP delisted.

04. Authentication Failures

Modern mail servers check SPF, DKIM, and DMARC records before accepting email. If these records are missing or misconfigured, the receiving server may reject or quarantine your messages.

SPF failure - Your domain's SPF record doesn't include the IP address of the server sending the email. This is common when using a third-party email service alongside server email.

DKIM failure - The DKIM signature on the message doesn't match, usually because the email was modified in transit or DKIM isn't configured for the domain.

DMARC failure - Both SPF and DKIM failed, and your DMARC policy says to reject such messages.

For setup instructions, see our email authentication guide.

05. Recipient-Side Issues

Sometimes the problem is entirely on the recipient's end:

Their mailbox is full - Nothing you can do except notify them through another channel.

Their spam filter is too aggressive - Ask them to whitelist your email address or domain.

Their domain's MX records are broken - If their domain's mail server isn't responding, all email to that domain will bounce. Check with our DNS lookup tool.

Their mail server is down - Temporary (4xx) bounces will retry automatically. Permanent (5xx) bounces won't.

06. Fixing Outbound Email Problems

1. Read the bounce message carefully - The error code and diagnostic text tell you exactly what's wrong
2. Check email authentication - Verify SPF, DKIM, and DMARC records are properly configured in your domain's DNS
3. Check for blacklisting - Use tools.ultrawebhosting.com to run a blacklist check on your mail server's IP
4. Check your email content - Avoid spam trigger words, excessive images, and URL shorteners in your messages
5. Check sending volume - Shared hosting accounts have outbound email limits to prevent abuse. If you're sending bulk email, consider a dedicated email service

Quick Recap: Fixing Email Bounces

1. Read the error code - 4xx = temporary (will retry), 5xx = permanent (needs a fix)
2. Check the address - 550 5.1.1 means the mailbox doesn't exist - check for typos
3. Check authentication - Missing SPF/DKIM/DMARC records cause rejections at major providers
4. Check blacklists - Run a blacklist check if the bounce mentions spam or reputation
5. Contact support - Forward the full bounce message to us for diagnosis

Last updated March 2026 · Browse all Email articles