Microsoft (Outlook.com and Microsoft 365) · permanent error
5.2.121: too many messages to one recipient this hour
The sender has gone over the number of messages Exchange Online lets one sender deliver to the same recipient within an hour.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.2.121
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
5.2.121 Recipient's per hour message receive limit from specific sender exceeded
What it means
Exchange Online limits how many messages a single sender can deliver to one recipient per hour. It exists so that one runaway automated sender cannot flood a mailbox. Once it is reached, further messages from that sender to that recipient are refused with 5.2.121, while other recipients are not affected. The admin of the receiving tenant can see the mailboxes involved in the Mailboxes exceeding receiving limits report.
Common causes
- An automated system sends a separate message to the same address for every event.
- A loop between two systems generates repeated messages to one recipient.
How to fix it
- 01Retry later; the limit applies per hour.
- 02Batch or digest automated notifications so fewer messages go to the same recipient.
- 03Recipient admins: use the Mailboxes exceeding receiving limits report to find the sender and the mailbox involved.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
Related codes
- 5.2.122Recipient has received too many messages this hour.The Microsoft 365 recipient has hit the hourly limit on messages it can receive from all senders combined, so new mail is refused for now.
- 432 4.3.2Recipient mailbox is receiving too fast.Exchange Online is slowing delivery to one mailbox because it is receiving too many messages at once; the sending server should retry later.
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