Basic SMTP reply code · RFC 5321
554 Transaction failed
The transaction failed; as the reply when the connection opens it means "no SMTP service here". Many providers use it for policy rejections.
- 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 554 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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554 5.4.6
Message exceeded 50 hops, this may indicate an email loop.
The message went through more than 50 servers, a sign of a forwarding loop. Check automatic forwarding.
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554 5.6.0
Email message is malformed. Not accepted.
The message does not follow the RFC 5322 format. Check with Header analyzer.
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554 5.7.0
Too many unauthenticated commands.
The client sent too many commands without authenticating.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026