Error 421 and 451: temporary failures, and why the message is delayed and not lost

An error that starts with a 4 (421, 450, 451, 452) is a “not now, try later” reply. It is not a refusal. The server that sent the message keeps it and tries again, by itself, for a good while. If at the end of that time it never went through, it returns a permanent-failure message (the subject of the article on the 550 error). In most cases the message does arrive: it just arrives later.

Why a server says “later”

Cause How to recognise it
Greylisting The destination refuses on purpose the first message from a sender it has never seen, and accepts the second try. It is a spam filter: robots do not usually try again. cPanel has the Configure Greylisting screen (Email section) where you see whether it is on for the account.
Rate limit Many messages quickly to the same destination. The big services answer 421 or 451 to anyone who looks like dumping mail.
Low reputation A new IP, or one with old complaints, is slowed down before being accepted.
The destination server is busy or under maintenance A short reply like “service not available”. It usually clears by itself.
A temporary DNS failure Something like “temporary lookup failure”. The DNS of the destination or of the sender did not answer in time.
The recipient’s mailbox is nearly full (452) “Insufficient storage”. If the mailbox is not emptied, it ends in a permanent refusal.

What to do, in order

1 Wait. A correct server tries again for hours, at growing intervals. You do not need to resend.
2 See whether the message stuck in your program. If it sits in the Outbox, your program could not hand it to the sending server, and the cause is another one: the sending server, the ports, the authentication. See why your e-mail is not sending or receiving.
3 Look at the trail of the sending. cPanel shows each attempt and the reply it received, in the Track Delivery screen. There you see the destination server’s exact text.
4 If the delay goes past a few hours, look for the cause: if the reply talks about “reputation”, “rate limit” or “blocked”, see your server IP on a blocklist and how to write an e-mail that does not land in spam.
5 If it is urgent, warn the recipient another way and ask them to confirm whether the message arrived, sometimes in the spam folder.
Do not send the same message ten times in a hurry. Each copy is another request to a server that asked you to wait. Besides not speeding things up, it adds to the look of bulk mail, and a domain that looks like it dumps mail is slowed down even more.
What if the delay is on this side? If your customers complain that the first e-mail from a stranger arrives late, it is the greylisting on this side doing its job. You can see and adjust the option under Configure Greylisting.

Has the message been delayed for many hours? Send us the destination address and the 421 or 451 line you saw.

Open a support ticket

SEE ALSO

Bounce message 550: what each one means

Track Delivery: finding out what happened to a message you sent

How to write an e-mail that does not land in spam

Why your e-mail is not sending or receiving

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