Should I retry a 450 error?

Yes, and your sending server does it automatically. 450 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.

How do I fix SMTP 450?

Let the retry happen. Greylisted mail normally goes through on the second attempt a few minutes later.

Temporary — will retry

SMTP 450

mailbox unavailable, try again later

SMTP 450 means the requested action was not taken because the mailbox is unavailable for now. RFC 5321 gives a busy mailbox or a temporary policy block as examples. It is a temporary failure, so the sending server retries. Greylisting and hostname checks are the most common reasons in practice.

How it appears

450 4.2.1 Mailbox temporarily unavailable

450 4.7.1 Client host rejected: cannot find your hostname

450 4.1.8 <bounce@example.com>: Sender address rejected: Domain not found

What causes it

The receiving server is greylisting you: it defers the first attempt from an unknown sender and accepts the retry.
Your sending IP has no reverse DNS, or the hostname does not resolve back to the IP.
The sender domain could not be resolved in DNS at that moment.
The mailbox is locked or busy on the receiving side.

How to fix it

In order — try the first one first.

Let the retry happen. Greylisted mail normally goes through on the second attempt a few minutes later.
Check that your sending IP has a PTR record and that the name resolves back to the same IP.
Check that the domain in your MAIL FROM resolves and has MX or A records.
If the same 450 repeats for days, read the text after the code. It usually names the check you are failing.

Common questions

What does SMTP 450 mean?: SMTP 450 means the requested action was not taken because the mailbox is unavailable for now. RFC 5321 gives a busy mailbox or a temporary policy block as examples. It is a temporary failure, so the sending server retries. Greylisting and hostname checks are the most common reasons in practice.
Should I retry a 450 error?: Yes, and your sending server does it automatically. 450 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
How do I fix SMTP 450?: Let the retry happen. Greylisted mail normally goes through on the second attempt a few minutes later.

Specific 450 replies

450 4.2.1 — mailbox temporarily unavailable

Last reviewed 2026-09-11