421: rate limit reached
Your server sent too many emails to IONOS in too short a time, so IONOS deferred the rest. Send fewer at once and try again later.
- IONOS
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421
- Provider
- IONOS (1&1)
- 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
421 Rate limit reached. Please try again later
What it means
IONOS limits how fast one sender may deliver to its customers. This 421 means your server went over that rate: too many emails in too short a time. It is temporary, and your server will retry the deferred messages. The IONOS help page gives the remedy plainly: reduce the number of emails sent at the same time, and restart sending later. The help page does not give the limit, so the safe approach is to spread volume out and raise it step by step while watching for further 421 replies.
Common causes
- A campaign or batch went out to many IONOS-hosted addresses at once.
- Your server opens many parallel connections to IONOS.
- A new IP or domain started sending at full volume.
How to fix it
- 01Lower the number of messages and connections your server sends to IONOS at the same time.
- 02Spread large sends over a longer period.
- 03Let the deferred messages retry later.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 421IP blocked for mail to unknown addresses.IONOS has blocked your server IP from sending to its customers for now, because too much of your mail went to addresses that do not exist.
- 421Too many messages in a short time.GMX received too many messages from your system in a short period and blocked it for a while. GMX can raise the limit on request.
- 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
- 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 [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.
- 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.
Further reading
All 17 IONOS codesEvery code in the wikiPaste your own bounce