451 4.7.500–699: temporary IP throttling
Microsoft 365 is deferring mail from the sending IP because its traffic changed sharply; the restriction lifts once the traffic looks legitimate.
- Microsoft
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451 4.7.500–699
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
451 4.7.500 Access denied, please try again later
What it means
Exchange Online returns codes from 4.7.500 to 4.7.699, often with an ASxxx reference, when a source IP suddenly sends far more than its history suggests. Microsoft treats new or changed senders with suspicion for a while, a form of graylisting it calls IP throttling, and slows them down while it evaluates the traffic. The code is temporary: a well-behaved server keeps retrying, and Microsoft says the restriction lifts shortly if the activity is valid. For senders whose own mail is not handled by Microsoft 365, the error clears as a sending history builds over a few days.
Common causes
- A new IP address started sending to Microsoft 365 recipients without any history.
- An established IP sent a much higher volume than usual.
- An on-premises server or third-party filter relays mail through Microsoft 365 without a connector set up for it.
How to fix it
- 01Let your server retry and keep volume steady rather than pushing harder; the deferrals ease as a history builds over a few days.
- 02Ramp new IPs up gradually instead of moving full volume to them at once.
- 03If your own server relays outbound mail through Microsoft 365, set up a connector from that server to Microsoft 365.
- 04If your inbound mail passes through a third-party service before Microsoft 365, set up a partner connector for it.
- 05Send only test messages from an onmicrosoft.com domain; add a custom domain for normal volume.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Fix NDR error code "451 4.7.500-699 (ASxxx)" in Exchange Onlinelearn.microsoft.com
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
- External senders: troubleshoot email sent to Microsoft 365learn.microsoft.com
Related codes
- 4.7.850–899Temporary restriction on the sending IP.Microsoft 365 detected suspicious activity on the sending IP and is restricting it temporarily while it evaluates the traffic.
- 421 [RP-001]Outlook.com rate limit for the sending IP.Outlook.com is limiting how fast your IP can send to it because of the IP or domain's reputation; it is temporary and your server should retry.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce