Bounce codes › Microsoft 365
550 5.2.121: Recipient's per hour message receive limit from specific sender exceeded
Microsoft 365 limits how many messages one sender may deliver to one mailbox per hour. You went over it for this recipient.
Wait and try again Recipient is getting too much mail
You sent this recipient more messages in an hour than Microsoft allows from one sender.
- Sent by: Microsoft 365 / Outlook
- Permanent (hard bounce) by its code, so your mail system will not retry by itself. The cause passes: a new message later can get through.
How Microsoft 365 words it (source):
Recipient's per hour message receive limit from specific sender exceeded
Why it happens
- An automated system (alerts, monitoring, a ticket tool) sent this person a burst of messages.
- A mail loop: two systems answering each other.
What to do
- Try again in an hour.
- Send this recipient fewer messages: combine alerts into one digest.
- If you did not mean to send so many, look for a loop or a script that got stuck.
Don't confuse it with
550 5.2.122 is the same limit counted across all senders: there the recipient is flooded by everybody, not just by you.
Questions
Is the address bad?
No. The mailbox exists and works. It only refused more mail from you this hour.
Why does Microsoft do this?
To stop one broken notification system from filling a person's inbox.
Got a different bounce?
Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.
Related codes
550 5.2.122Recipient's per hour message receive limit exceeded (Microsoft 365)550 5.2.2Submission quota exceeded (Microsoft 365)