What does 553 mean?

You fire off an email and get back a 553. Unlike a soft bounce that might clear up on its own, a 553 is a permanent rejection. The receiving server is telling you, clearly, that the address name itself is the problem.

Technically it means "Requested action not taken: mailbox name not allowed." The server received your message but refused to accept it because something about the recipient address (or sometimes the sender address) doesn't meet its rules.

The most common triggers are:

• Malformed address format: Something like captain@@deepcurrent.io, kraken @thedeepsea.com (space before the @), or lighthouse#harborpost.net (wrong separator). A valid email has exactly one @ with no spaces and a proper domain after it.
• Unsupported characters: Some servers reject addresses with characters like !, #, or % in the local part, even though the spec technically allows some of them.
• Policy-based blocks: Certain servers refuse to accept mail sent to role addresses like noreply@, postmaster@, or admin@ depending on their configuration.
• Relay denied: You're trying to send through a server that won't relay mail for your domain, which often shows up with a 553 message that mentions relay access denied.

Common message strings you might see include "553 Mailbox name not allowed", "553 Invalid recipient", and "553 Requested action not taken: mailbox name not allowed".

A 553 is a permanent 5xx failure, which means retrying won't help. Suppress the address immediately. Don't keep sending to it hoping the server changes its mind.

If you're seeing 553 errors across a batch of addresses, it's worth running a validation pass on your list before your next send. A stale list with badly formatted addresses will keep generating hard bounces and hurt your sender reputation over time. If that's your situation, RME Clean can sort through and flag anything that shouldn't be sent to.