Enhanced SMTP status code · RFC 3463 · IANA
5.3.2 System not accepting messages
The server is not accepting messages, for example ahead of a shutdown, under heavy load or during maintenance.
- 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 can be temporary or permanent.
Also seen as:
4.3.2 - Subject: X.3 · Mail system
Something about the destination server; under its administrator's control.
- Associated basic codes (IANA registry)
How 5.3.2 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
-
432 4.3.2
STOREDRV.Deliver; recipient thread limit exceeded
The recipient's mailbox is getting too many messages too quickly and is throttled for a while, so it does not affect others. The sending server retries.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026