421 Service not available, closing transmission channel
The server is ending the session and wants you to come back later. 421 is the SMTP reply for "not now": the server may be shutting down, overloaded, out of connection slots, or deliberately dropping a client that misbehaved. Your MTA keeps the message and retries on its normal schedule.
What the server replies
421 4.3.2 Service not available, closing transmission channel
- Sent by
- Any mail server
- Type
- TemporaryRetry helps
- About
- Connection
- Enhanced code
- 4.3.2
Who sends it
Any SMTP server. Postfix sends it when a client exceeds smtpd_client_connection_count_limit or when the server is stopping; Exim when it hits smtp_accept_max; Gmail as "421 4.7.0 Try again later, closing connection". Variants with 4.4.2 ("Timeout exceeded") mean your side went quiet for too long.
Why it happens
- The receiving server is at its connection limit or under heavy load.
- Your server opened too many parallel connections to the same host.
- The server is restarting or in maintenance.
- Your client stalled mid-session and hit the server's idle timeout (the 4.4.2 form).
- A reputation-based early disconnect, which some providers do instead of a 5xx.
How to fix it
Let the retry happen
A single 421 needs no action. Check again after an hour; if the queue is still deferring, read the exact text, because the enhanced code narrows the cause.
Reduce concurrency
Set a per-destination connection limit of a few connections and reuse each for a batch of messages. Receivers prefer fewer, longer sessions.
Test the server directly
Connect on port 25 with an SMTP test. If the banner arrives and EHLO works, the server is up and the 421 was a per-client limit; if the connection itself fails, it is down or firewalled.
Fix timeouts on your side
For 4.4.2 timeouts, look for slow DNS, a content filter that holds the DATA phase, or a proxy that delays the connection.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 451 4.3.0 Temporary system problem / temporary lookup failureSomething inside the receiving server failed while it was handling your message and it is asking you to retry.
- 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP addressGmail is throttling the sending IP.
- 4.4.7 Message delayed, QUEUE.Expired; message expiredExchange tried to deliver the message for as long as it is allowed to (two days by default in Exchange Online) and gave up.
Sources
Fewer bounces to look up.
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