Microsoft (Outlook.com and Microsoft 365) · temporary error
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.
- Microsoft
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421 [RP-002]
- 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-002 is the per-connection form of the Outlook.com rate limit: the connecting IP exceeded the rate allowed on that connection, and as with RP-001, Microsoft says the reason is related to IP or domain reputation. Because it is a 421, the message stays in your queue for a later attempt. Sending fewer messages per connection and spreading them over time keeps you under the limit while reputation improves.
Common causes
- Your server sends many messages down one connection to Outlook.com.
- The IP or domain reputation limits what Outlook.com accepts on each connection.
How to fix it
- 01Reduce the number of messages your server sends per connection to Outlook.com.
- 02Retry deferred messages with backoff.
- 03Watch the IP in SNDS and act on complaint reports from JMRP.
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-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.
- 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.
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