Enhanced SMTP status code · RFC 3463 · IANA
5.3.0 Mail system problem
The destination server exists and normally accepts mail, but something about it caused the error.
- 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).
Also seen as:
2.3.0,4.3.0 - Subject: X.3 · Mail system
Something about the destination server; under its administrator's control.
How 5.3.0 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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451 4.3.0
Email server has temporarily rejected this message.
Gmail temporarily rejected the message, with no other reason. The sending server's automatic retry usually solves it.
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451 4.3.0
Multiple destination domains per transaction is unsupported. Please try again.
The sending server put recipients from several domains in one transaction, which Gmail does not accept; it needs one transaction per domain.
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421 4.3.0
Temporary System Problem. Try again later.
Gmail temporarily rejected the message, with no other reason. The sending server's automatic retry usually solves it.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026