Types of email bounces

When an email cannot be delivered, the sending server returns a bounce message with a code that describes why. Understanding these codes helps you diagnose and fix delivery problems.

Bounce types

Address change (bounce_ac)

The recipient has changed their email address and their old account is sending an automatic reply to notify senders of the new one. Update your records with the new address.

Auto-reply (bounce_ar)

Auto-replies - typically out-of-office notifications - are sent by the recipient's email client. They do not mean delivery failed; they simply indicate the recipient is temporarily unavailable. This matters when you are sending time-sensitive information, as the recipient may not see your message until they return.

Challenge/response (bounce_cr)

Some recipients use filtering software that only accepts mail from known senders. When your message arrives, the filter sends you a challenge - usually asking you to click a link or complete a simple action - to confirm you are a real sender. If you do not complete the challenge, your original message is not delivered.

DNS failure (bounce_df)

The receiving server cannot deliver your message because of a temporary DNS problem. This is usually transient; the sending server will retry automatically. If it persists, the domain's DNS records may be misconfigured.

Hard bounce (bounce_hb)

A hard bounce means the message was permanently rejected. Common causes are:

• The recipient's domain does not exist.
• The recipient's email address is unknown or has been deleted.

You should remove hard-bouncing addresses from your mailing list, as continuing to send to them can harm your sender reputation.

Email blocked (bounce_mb)

The recipient's mail server is actively blocking email from your server. You may see error messages such as:

550 Message REFUSED by peer
552 Blocked by filters

This can happen if your server's IP address is on a blocklist. Check your IP reputation and, if necessary, contact your hosting provider for assistance.

Mailbox full (bounce_mf)

The message could not be delivered because the recipient's mailbox has reached its storage limit. This is a temporary condition - delivery may succeed once the recipient frees up space.

Soft bounce (bounce_sb)

A soft bounce means the message reached the recipient's mail server but was returned before final delivery. The most common cause is a full inbox, but it can also result from a temporary server issue. The sending server will retry delivery automatically. After several failed attempts, a soft bounce is typically reclassified as a hard bounce.

Transient bounce (bounce_tr)

A transient bounce is generated by the sending server to let you know that a message has not yet been delivered but that the server is still trying. You may see a notice such as:

Warning: message still undelivered after 4 hours. Will keep trying until message is 2 days old.

In most cases you can safely ignore a transient bounce and wait for the server to retry. If the message is still undelivered after the retry window expires, it will become a hard bounce.