Email sending failure: Bounce mail

When an email cannot be delivered, it “bounces” back to you with an error message. Bounces are an unavoidable part of email sending, especially for bulk or mass messages, and each bounce notification includes a code explaining why it happened.

While occasional bounces are normal, repeatedly sending emails to addresses that bounce can harm your sender reputation and signal to email providers that your messages may be unwanted or spam.

This guide will review the most common bounce codes and how to handle them so your messages can get through.

What is a bounce error message?

A bounce error message is an automated reply from the recipient’s email server explaining why your message was not delivered.

It usually contains:
• A status code (e.g., 550, 421, 5.1.1)
• A short description of the problem
• Sometimes a suggestion for how to fix it

These codes are sometimes called SMTP reply codes or email bounce codes.

Common email bounce codes

550 – Hard Bounce: The recipient’s email address does not exist or cannot receive mail.
552 – Hard Bounce: The recipient’s mailbox is full and cannot accept new messages.
554 – Hard Bounce: Message rejected by the recipient server (often due to spam filters or blocklists).
421 – Soft Bounce: Temporary server issue, try again later.
450/451/452 – Soft Bounce: The recipient’s server is busy, the mailbox is temporarily unavailable, or the storage limit has been reached.
5.1.1 – Hard Bounce: Email address not found (invalid or misspelt).

Hard Bounce versus Soft Bounce

Hard Bounce: Permanent failure. The message will never be delivered unless you correct the issue (e.g., wrong email address).

Soft Bounce: Temporary failure. Delivery might succeed if retried later (e.g., the recipient’s mailbox is temporarily full).

How to handle bounce mail?

1. Read the bounce error message carefully
• Look for the SMTP code and description to understand the cause.

2. If it is a hard bounce
• Check for typos in the recipient’s email address.
• Remove invalid addresses from mailing lists.
• If the domain no longer exists, contact the recipient via another method.

3. If it is a soft bounce
• Wait and try sending again later.
• If the issue persists, contact the recipient to check their mailbox or server.

4. Check your own sending reputation
• Avoid spam-like subject lines or content.
• Ensure your domain has proper SPF, DKIM, and DMARC records.
• Verify you are not on an email blocklist.

5. For bulk senders
• Use a reputable email marketing platform that manages bounce handling automatically.