5.2.121 Microsoft bounce: per-sender hourly limit
What 5.2.121 means
One sender sent more messages per hour to a single recipient than Exchange Online allows.
Message text
Recipient's per hour message receive limit from specific sender exceeded
Common causes of 5.2.121
- Automated mailer or sender exceeded the per-hour limit for one recipient
How to fix 5.2.121
- Lower the number of messages per hour sent to that recipient and retry later.
- Route automated notifications through a sender that spreads volume across recipients.
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How to read 5.2.121
The second part, 5.2.121, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 2 is mailbox: a problem with the recipient mailbox, such as full, disabled or unknown. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 5.2.2 SMTP: mailbox full
- 5.2.3 SMTP: message too large
- 5.2.2 Microsoft: submission quota exceeded
- 5.2.122 Microsoft: recipient hourly receive cap
- 550 5.2.1 Gmail: recipient mailbox inactive
- 552 5.2.2 Gmail: inbox full and inactive
FAQ
What does 5.2.121 mean?
One sender sent more messages per hour to a single recipient than Exchange Online allows. This is a permanent failure.
How do I fix 5.2.121?
Lower the number of messages per hour sent to that recipient and retry later. Route automated notifications through a sender that spreads volume across recipients.
Is 5.2.121 permanent, and should I retry?
5.2.121 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.