421 SMTP bounce: service not available
What 421 means
The receiving server is shutting down, overloaded or has refused the connection for now, so it closes the session. The sender should retry later.
Message text
<domain> Service not available, closing transmission channel
Common causes of 421
- Receiving server is restarting, shutting down or overloaded
- Too many simultaneous connections from your sending IP
- Receiver has temporarily throttled or blocked your IP or domain
How to fix 421
- Wait; the sending queue retries automatically and most 421s clear within hours.
- Confirm the recipient's MX hosts answer with MX lookup.
- If it continues for more than a day, check your sending IP with blocklist check.
- Check that your server's PTR record matches its HELO name with reverse DNS lookup.
Check your domain
Check the MX records of the recipient domainRuns in your browser; nothing is uploaded.
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 421
The first part, 421, is the SMTP reply code. Service not available, closing transmission channel: the server is shutting down or refusing the connection for now. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
This reply carries no enhanced status code. The three-digit reply code is the only machine-readable signal, so read the text that follows it to learn the reason.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: www.rfc-editor.org. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 450 SMTP: mailbox unavailable
- 451 SMTP: local processing error
- 452 SMTP: insufficient storage
- 4.2.2 SMTP: mailbox full
- 4.4.1 SMTP: no answer from host
- 4.7.0 SMTP: security status
FAQ
What does 421 mean?
The receiving server is shutting down, overloaded or has refused the connection for now, so it closes the session. The sender should retry later. This is a temporary failure.
How do I fix 421?
Wait; the sending queue retries automatically and most 421s clear within hours. Confirm the recipient's MX hosts answer with MX lookup. If it continues for more than a day, check your sending IP with blocklist check. Check that your server's PTR record matches its HELO name with reverse DNS lookup.
Is 421 permanent, and should I retry?
421 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.