550 5.2.121: Microsoft per sender receive limit hit
550 5.2.121Per hour receive limit from one sender exceededPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com
550 5.2.121 Recipient's per hour message receive limit from specific sender exceeded
What it means
Exchange Online caps how many messages a single recipient mailbox will accept from one sender in an hour. When your address goes over that number, further messages to that mailbox are rejected with 5.2.121 until the hour rolls over. The text typically reads like the example. A sibling code, 5.2.122, covers the limit across all senders combined.
This is a protection for the recipient, not a judgement about your reputation. You are not blocked from the rest of the tenant, only from that one mailbox for a short while. It usually shows up with automated senders: monitoring alerts, ticket systems, and scanners that send one message per event.
Likely causes
- An alerting system sending a separate message for every event to one on call mailbox.
- A mail loop between an autoresponder and a ticket system that bounces messages back and forth.
- A batch job that emails a report per record to the same person.
- A misconfigured forward sending every copy of a busy shared mailbox to one user.
How to fix it
- Find the source of the high volume to that mailbox in your sending logs.
- Batch alerts into digests or throttle the application to a few messages per minute.
- Break any loop with an autoresponder by suppressing replies to automated mail (look for Auto-Submitted headers).
- If the volume is legitimate, spread it across several recipient mailboxes or deliver it to a channel built for alerts rather than email.
Checks that apply
- Email Header Analyzer Paste one of the many messages to spot Auto-Submitted or loop indicators that reveal an autoresponder ping pong.
- MailFlow Monitor Confirms normal delivery to the mailbox resumed after you reduced the rate.
FAQ
- How long does the block last?
- The limit is per hour, so delivery from your address to that mailbox recovers once the rate drops back under the cap.
- Does it affect other recipients in the same company?
- No. The count is per recipient mailbox and per sender, so other mailboxes in the tenant still accept your mail.