5.2.121: Recipient’s hourly receive limit exceeded
Exchange Online returns 5.2.121 when you sent more messages per hour to one Microsoft 365 recipient than it allows from a single sender, and 5.2.122 when that recipient hit its hourly limit from all senders. Wait, then send to that recipient less often.
The exact messages
Microsoft 365 / Exchange Online
5.2.121 Recipient's per hour message receive limit from specific sender exceeded
Microsoft 365 / Exchange Online
5.2.122 Recipient's per hour message receive limit exceeded
Why it happens
- An automated system sending many messages to one recipient.
- A mail loop or notification storm.
How to fix it
- Wait and resend later.
- Reduce how many messages per hour go to that one recipient.
If you send cold email
Outreach rarely hits this unless a sequence misfires and sends repeatedly to the same contact. Check for duplicate contacts in the list.
How WarmHawk handles it
WarmHawk never sends more than a mailbox’s own daily cap, spaces sends from one mailbox at least 8 minutes apart with random jitter, and starts a newly warmed mailbox at 5 campaign emails a day, growing 20% a day. A temporary failure is retried with backoff (30 minutes, then doubling) and the lead is suppressed after 4 attempts, so a throttled mailbox is never hammered. How WarmHawk works →
Related bounce codes
Sources, checked 2026-09-29: Microsoft Learn: NDRs and SMTP errors in Exchange Online. Have a different bounce? Paste it into the decoder →
Questions
5.2.121 questions
What does 5.2.121 mean?+
Exchange Online returns 5.2.121 when you sent more messages per hour to one Microsoft 365 recipient than it allows from a single sender, and 5.2.122 when that recipient hit its hourly limit from all senders. Wait, then send to that recipient less often.
Is 5.2.121 a temporary or permanent error?+
Permanent. The message will not be retried, and sending it again unchanged will fail the same way until the cause is fixed.
How do I fix 5.2.121?+
Wait and resend later. Reduce how many messages per hour go to that one recipient.