421: too many parallel connections
Your server opened too many simultaneous connections to the t-online.de mail servers, so the extra ones were refused for now and the mail is delayed.
- Telekom
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421
- Provider
- Telekom (t-online.de)
- 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 Maximum parallel connections for your IP-Address reached
What it means
Telekom limits how many connections one IP may hold to its mail exchangers at once. This 421 means your server already had several open, all transferring mail, and tried to open more. It is temporary, but if it happens often your customers notice delays to t-online.de addresses. The postmaster page asks you to configure your server so it does not open a new connection for every message, and does not needlessly cap how many messages go over one connection. The page names the settings to look for: peer concurrency and destination concurrency.
Common causes
- Your server opens a new connection for each message.
- Concurrency toward t-online.de is set high, or several servers share one IP.
- The number of messages allowed per connection is set very low.
How to fix it
- 01Lower the number of simultaneous connections to t-online.de mail servers.
- 02Send more messages over each connection instead of opening new ones.
- 03Check the peer and destination concurrency settings of your mail server.
- 04Let the deferred messages retry.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- t-online.de Postmaster (Deutsche Telekom)postmaster.t-online.de
Related codes
- 421Too many connections from your IP.Your server opened more simultaneous connections to a GMX mail server than one IP may hold. Wait for an open connection to close, then connect again.
- 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.
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