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.
- Microsoft
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421 [RP-001]
- 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 it means
RP-001 comes from Outlook.com, the consumer mail service behind outlook.com, hotmail.com, live.com and msn.com addresses. It means the connecting IP has exceeded the rate limit Outlook.com allows it, and Microsoft ties that limit to IP and domain reputation. As a 421 reply it defers rather than rejects: your server keeps the mail and retries. Persistent RP-001 means the reputation does not support the volume you send, so the answer is slower sending and better reputation, which SNDS lets you watch.
Common causes
- The IP sends more mail to Outlook.com than its reputation supports.
- The IP is new or recently ramped up and has little history.
- Junk complaints from Outlook.com users have lowered the IP or domain reputation.
How to fix it
- 01Let your server retry with backoff instead of reconnecting immediately.
- 02Lower the sending rate to Outlook.com, and ramp new IPs up gradually over a couple of weeks.
- 03Enroll the IP in SNDS to watch its reputation, and in JMRP to receive complaint reports.
- 04Send only to engaged, opted-in recipients and remove anyone who complains.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Outlook.com Postmaster: Troubleshooting (SMTP error codes)substrate.office.com
Related codes
- 421 [RP-002]Outlook.com rate limit on this connection.Your IP has sent more over a single connection than Outlook.com allows, a limit Microsoft ties to IP and domain reputation.
- 421 [RP-003]Outlook.com connection limit.Your IP has opened more connections to Outlook.com than it is allowed, a limit Microsoft links to IP and domain reputation.
- 421Service not available.The receiving server is closing the connection and will not take mail right now. The reply is temporary, so the sending server keeps the message and tries again later.
421 at other providers
- 421Temporary error or deferral.Yahoo or AOL refused your message for now with a 421, one of its two temporary error codes. Your server should keep it and retry; repeated deferrals point to reputation or complaints.
- 421Try again later.Xfinity turned the message away for now with a generic 421. Treat it as temporary and let your server retry: Xfinity uses it for several reasons it does not spell out.
- 421Reverse DNS lookup failed.Xfinity could not complete a reverse DNS lookup for your sending IP. It treats this as temporary, so your server should retry, but a broken DNS setup keeps it failing.
- 421Temporary reject under an exceptional blocklist.GMX refused the connection from your server for now under one of the blocklists it applies in exceptional situations. Your server will retry; GMX asks you to stop the misuse first, then contact it.
- 421Too much of your mail is spam.GMX is refusing mail from your server for a while because too large a share of its recent mail was classed as spam by the GMX filters.
- 421Too many mails to addresses that do not exist.GMX is turning your server away for a while because it sent too much mail to GMX addresses that do not exist. Clean the list before sending again.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce