5.2.121 and 5.2.122: recipient's per hour message receive limit exceeded
The Microsoft 365 recipient received more messages in an hour than Exchange Online allows, either from you alone (5.2.121) or from all senders combined (5.2.122).
Recipient's per hour message receive limit from specific sender exceeded
Microsoft's text for 5.2.121 in its Exchange Online NDR reference; 5.2.122 reads "Recipient's per hour message receive limit exceeded".
What 550 5.2.121-122 means
Microsoft's NDR reference lists two receiving-limit codes. 5.2.121, "Recipient's per hour message receive limit from specific sender exceeded", means the sender went over the number of messages they may send per hour to one Exchange Online recipient. 5.2.122, "Recipient's per hour message receive limit exceeded", means the recipient went over the number of messages they can receive per hour from all senders.
Microsoft's Exchange Online limits page sets the receiving limit at 3,600 messages per hour per mailbox, and the limit for messages from a single sender at 33 percent of that. When the limit is exceeded, mail from the internet or on-premises senders is returned with an NDR, and after an hour the limit refreshes. Microsoft says the limits protect users from inboxes filling up from errant automated notification systems and other mail storms.
So although the code starts with 5, Microsoft's advice is that the automated mailer or sender should try again later and reduce how many messages it sends per hour to that recipient. Common triggers are monitoring systems, ticket tools and scripts stuck in a loop. Admins can see affected mailboxes in the Mailboxes exceeding receiving limits report.
Likely causes
- An automated system, such as monitoring, ticketing or a script, sending many messages to one mailbox.
- A mail loop between an auto-reply, a forward and another system.
- A shared or role mailbox receiving a surge of mail from many senders at once.
How to fix it
If you sent the message
- Slow down: send fewer messages per hour to that recipient, or batch alerts into digests.
- Fix loops between auto-replies, forwards and ticket systems.
- Resend important messages after an hour, once the limit has refreshed.
If you're the recipient or their admin
- Admins: check the Mailboxes exceeding receiving limits report to find which mailbox and sender are involved.
- Point heavy automated mail at a mailbox or system built for that volume.
Frequently asked questions
How many messages per hour can a Microsoft 365 mailbox receive?
Microsoft's Exchange Online limits page sets the receiving limit at 3,600 messages per hour, with messages from a single sender limited to 33 percent of that. The limit refreshes after an hour.
Should I resend after a 5.2.121 bounce?
Yes, but later and more slowly. Microsoft says the sender should try again later and reduce the number of messages it sends per hour to that recipient.
Sources
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn)
- Exchange Online limits: receiving limits (Microsoft Learn)
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
- 554 5.4.6 554 5.4.6: routing loop detected
- 450 SMTP 450: requested mail action not taken, mailbox unavailable
- 550 5.4.5 550 5.4.5: daily user sending limit exceeded
- 452 4.2.2 452 4.2.2: the recipient's inbox is out of storage space
- 554 5.2.0 554 5.2.0 STOREDRV.Deliver.Exception: cannot get ID from name
- 550 5.7.512 5.7.512: access denied, message must be RFC 5322 section 3.6.2 compliant
- 550 5.4.8 5.4.8: MX hosts of the domain failed MTA-STS validation
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