Basic SMTP reply code · RFC 5321
553 Mailbox name not allowed
The action was not taken: the mailbox name is not allowed, for example its syntax is wrong.
- Class: 5yz · Permanent failure
5yz: a permanent failure. The same request should not be repeated; something must change first, such as the address (RFC 5321 §4.2.1). - Source
How 553 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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553 5.1.2
We weren't able to find the recipient domain. Please check for any spelling errors and make sure you didn't enter any spaces, periods, or other punctuation after the recipient's email address.
The recipient's domain was not found. Check its spelling. Check with Email DNS check.
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553 5.1.3
The recipient address address is not a valid RFC 5321 address.
The recipient's address is not a valid RFC 5321 address.
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553 5.1.7
The sender address address is not a valid RFC 5321 address.
The sender's address (MAIL FROM) is not a valid RFC 5321 address.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026