Enhanced SMTP status code · RFC 3463 · IANA
5.2.121 Recipient's per hour message receive limit from specific sender exceeded
The sender sent the same recipient more messages per hour than Exchange Online allows.
- 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).
- Subject: X.2 · Mailbox
Something about the recipient's mailbox; under the recipient's control.
How 5.2.121 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
-
5.2.121
Recipient's per hour message receive limit from specific sender exceeded
The sender sent the same recipient more messages per hour than Exchange Online allows.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026