421: try 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.
- Comcast
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 421
- Provider
- Comcast (Xfinity)
- 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 Try again later
What it means
Xfinity uses this generic 421 for several reasons and does not list them. It asks senders to treat it as a temporary failure and to try again later, which a correctly set up mail server does by itself. A single occurrence needs no action. If it keeps coming back for the same server, look at the conditions Xfinity does document: its rate limits on inbound mail, which carry RL codes, its reverse DNS requirement, which has its own wording, and its limit of 25 simultaneous connections per sending IP address, whose reply its page does not show.
Common causes
- Xfinity's servers were busy or limiting traffic when your server connected.
- Your server may have opened more than the 25 simultaneous connections per IP that Xfinity allows, a limit whose reply Xfinity does not show.
- A temporary condition on the receiving side that Xfinity does not describe.
How to fix it
- 01Let your server retry on its normal schedule.
- 02Cap simultaneous connections to Xfinity at 25 or fewer per sending IP.
- 03If the error persists, check the full reply for an RL code or a reverse DNS message and follow that entry.
- 04If you need more help, use Report an issue under Postmaster on Xfinity's Customer Security Assurance site.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- RL000001Rate limited by sender reputation.Xfinity is slowing mail from your server with a temporary 4xx because it passed the hourly recipient limit Xfinity sets from your SenderScore. Your server should retry later.
- 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.
- 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.
- 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.
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