SMTP 450 4.2.0: mailbox unavailable (temporary, often greylisting)
A 450 4.2.0 reply means the recipient mailbox is temporarily unavailable, so the message is deferred rather than bounced. RFC 3463 defines X.2.0 as "Other or undefined mailbox status", and the 4.x form tells your server to retry. Greylisting and short-lived mailbox conditions are the usual reasons.
The enhanced status code 4.2.0 is defined by RFC 3463 as "Other or undefined mailbox status": a mailbox-level condition the server did not describe further. Paired with the 450 basic code, it is a temporary defer, so the mailbox exists and the receiver wants you to try again.
Common reasons
Greylisting: an unknown sender is deferred once and accepted on retry.
The mailbox is briefly locked, for example during maintenance or a quota recount.
A transient filtering or storage condition on the recipient side.
What to do
Let your server retry, which it does automatically for a 4.x code. Greylisted mail typically lands on the second attempt a few minutes later. If the defers continue past the retry window, read the reply text and check your sender reputation. The permanent counterpart is 550 5.2.0, which the receiver returns when it decides to stop retrying.
Common questions
Is 450 4.2.0 a permanent bounce?→
No. The 4 marks it transient. The mailbox exists and your server retries automatically.
Why is greylisting so often the cause?→
Greylisting defers an unfamiliar sender on purpose and expects a retry. Legitimate servers retry and are accepted; many spam sources do not.
How long until 450 4.2.0 clears?→
Greylisting usually clears on the next retry a few minutes later. Other transient conditions clear once the mailbox is available again.
What is the permanent version?→
550 5.2.0. The receiver returns it when it stops retrying and treats the mailbox condition as a hard failure.