Microsoft 365 5.2.121
Recipient's per hour message receive limit from specific sender exceeded
Microsoft 365 5.2.121 means: recipient's per hour message receive limit from specific sender exceeded. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Recipient's per hour message receive limit from specific sender exceeded
Possible cause
The sender exceeded the maximum number of messages that they're allowed to send per hour to a specific recipient in Exchange Online.
Provider's advice
The automated mailer or sender should try again later, and reduce the number of messages that they send per hour to a specific recipient. This limit helps protect Microsoft 365 and Office 365 users from rapidly filling their Inboxes with many messages from errant automated notification systems or other single-sender mail storms. The "Mailboxes exceeding receiving limits" report shows detailed information about the mailboxes that are receiving large volumes of messages in a short time.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.2.121: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.2.0 other or undefined mailbox status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
This limit protects a single Exchange Online recipient from a flood of messages from one sender, often an automated system sending alerts or reports. Messages are retried, but sustained sends over the limit keep failing. Batch the messages into a digest or slow the automated sender. The recipient address is valid.
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 5.2.121?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Microsoft 365 5.2.121 mean?
- Microsoft 365 5.2.121 means: recipient's per hour message receive limit from specific sender exceeded. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 5.2.121?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Would verifying my list have prevented Microsoft 365 5.2.121?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05