Enhanced SMTP status code · RFC 3463 · IANA
5.4.5 Mail system congestion
The message could not be delivered because the mail system was congested.
- 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 temporary failure.
Also seen as:
4.4.5 - Subject: X.4 · Network and routing
Network and routing status (DNS included); under the control of the destination's or an intermediate server's administrator.
- Associated basic codes (IANA registry)
How 5.4.5 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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550 5.4.5
Daily user sending limit exceeded.
The sending account reached Gmail's daily sending limit.
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550 5.4.5
Daily SMTP relay limit exceeded for user.
The daily limit of Google Workspace's SMTP relay service was reached for this user or for the organisation.
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421 4.4.5
Server busy, try again later.
Gmail's servers are busy; a later retry solves it.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026