Microsoft (Outlook.com and Microsoft 365) · temporary error
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.
- Microsoft
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421 [RP-003]
- 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-003 means the connecting IP has exceeded the connection limit Outlook.com allows it, and Microsoft relates the limitation to IP and domain reputation. Separately, its sender policies say senders must not open more than 500 simultaneous connections to Outlook.com inbound servers without prior arrangement. The 421 is temporary, so the mail is retried; opening fewer parallel connections and backing off between attempts usually clears it.
Common causes
- Your server opens many parallel connections to Outlook.com.
- The IP or domain reputation lowers the number of connections Outlook.com accepts from it.
How to fix it
- 01Lower the number of simultaneous connections your server opens to Outlook.com, staying well under 500.
- 02Retry deferred mail with backoff rather than opening new connections.
- 03Monitor the IP in SNDS and keep complaint rates low.
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
- Outlook.com Postmaster: Policies, practices, and guidelinessubstrate.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-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 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