Enhanced SMTP status code · RFC 3463 · IANA
5.1.3 Recipient address syntax is invalid
The recipient's address is not syntactically valid.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code is only useful as a permanent failure.
- Subject: X.1 · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
- Associated basic codes (IANA registry)
How 5.1.3 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 5.1.3
The recipient address address is not a valid RFC 5321 address.
The recipient's address 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