A bounce-back message is an automated reply from a mail server telling you that your email didn't reach the person you sent it to. Rather than guessing what went wrong, you can usually read the bounce message itself to find the exact reason and fix it.
Hard Bounces vs Soft Bounces
Bounces fall into two broad categories, and knowing which one you're dealing with tells you whether to try again or fix something first.
- Hard bounce: a permanent failure. The address doesn't exist, the domain is wrong, or the recipient's server has permanently rejected the message. Resending won't help until the underlying problem is fixed.
- Soft bounce: a temporary failure. The recipient's mailbox is full, their server is briefly down, or your message got rate-limited. These often resolve on their own, and your mail server will usually retry automatically for a while before giving up.
Common Bounce Messages and What They Mean
Most bounce-back emails include a numeric SMTP status code and a short explanation. Here are the ones you'll see most often.
- 550 5.1.1 User unknown: the mailbox doesn't exist on the recipient's server. Double-check the address for typos, or ask the recipient to confirm it's still active.
- 552 5.2.2 Mailbox full: the recipient's inbox has reached its storage limit. This is on their end, not yours. Try again later or reach them another way.
- 550 5.7.1 Relay access denied or spam blocked: the receiving server rejected your message as likely spam, or your server isn't authorized to send on behalf of your domain. This is usually an SPF, DKIM, or blacklist issue on your side.
- 421 or 450 Temporary failure: the receiving server asked your mail server to try again later. This is normal and usually clears within a few minutes to a few hours.
- 554 Message rejected: the recipient's server refused the message outright, often because of a strict spam filter or an attachment it didn't like.
The exact wording varies by mail provider, but the three-digit code (e.g. 550) and the extended code after it (e.g. 5.1.1) are fairly standard across servers, so you can look up an unfamiliar one for more detail.
What to Do When You Get a Bounce-Back
- Read the full bounce message, not just the subject line. The exact reason is usually named near the bottom of it.
- If it mentions an unknown user or invalid address, check the recipient's email address for typos and confirm it with them another way if you can.
- If it mentions spam, blacklisting, or relay access denied, check whether your sending domain or server IP has ended up on a spam blacklist.
- If it's a temporary failure (soft bounce), wait. Most mail servers automatically retry for 24 to 48 hours before giving up permanently.
- If bounces keep happening for the same recipient, contact them directly by phone or another email address to confirm the correct address or ask them to check their mailbox.
If you're getting bounce-backs on messages you never sent, or a sudden flood of bounces all at once, your email account may have been compromised and used to send spam. See how to detect and stop a compromised email account for what to check and how to lock it down.