How long until a 450 becomes a bounce?

It depends on the sending server’s queue lifetime, commonly 4 to 5 days. After that the sender gets a final bounce, usually with 4.4.7.

Is greylisting safe to ignore?

Yes, as long as your server retries normally. Greylisting only delays the first message from a new sender.

SMTP 450: mailbox unavailable (temporary)

450Requested mail action not taken: mailbox unavailableTemporary: the sender retries

Typical wording

450 4.2.0 <user@example.com>: Recipient address rejected: Greylisted

What it means

RFC 5321 calls 450 "Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)". It usually answers a RCPT TO command, so it refuses one recipient while the rest of the session carries on.

In practice the most common source is greylisting: the receiver defers the first attempt from an unknown sender and accepts the retry a few minutes later. Others are a locked or busy mailbox, a temporary DNS failure while checking your sender domain, or a policy service that could not answer in time.

A 450 never bounces on the first try. Your server queues the message and retries. It differs from 550, which is the permanent form of the same refusal, and from 451, which reports a local processing error rather than a problem with the mailbox.

Likely causes

Greylisting on the receiving server, which defers the first delivery from a new sender, IP, and recipient combination.
The receiver could not resolve your sender domain or your HELO name at that moment and deferred instead of rejecting (Postfix uses 450 4.1.8 for this).
The recipient mailbox is locked, being migrated, or busy with another delivery.
A rate or reputation policy that temporarily blocks your IP for that recipient.

How to fix it

Wait for the retry. Greylisting usually clears on the second attempt, typically 5 to 30 minutes later, so do not treat a single 450 as a failure.
Make sure your MTA retries from the same IP. Sending pools that rotate IPs on every attempt can keep tripping greylisting forever.
If the text mentions your domain or hostname, check that your sender domain has working MX or A records and that your HELO name resolves.
If 450 repeats for days, contact the recipient’s admin with the full reply line and timestamp; they can whitelist you in their greylisting or policy service.

Checks that apply

Your domain or sending IP (fills in the checks below)

MX Lookup Shows the recipient domain’s MX hosts so you know which server deferred you and can quote it to the right admin.
Reverse DNS (PTR) Checks the reverse DNS of your sending IP; missing PTR records often turn a quick greylist pass into repeated 450 replies.
DNS Server Audit Audits your domain’s nameservers, catching slow or lame servers that make receivers defer with 450 4.1.8.

FAQ

How long until a 450 becomes a bounce?: It depends on the sending server’s queue lifetime, commonly 4 to 5 days. After that the sender gets a final bounce, usually with 4.4.7.
Is greylisting safe to ignore?: Yes, as long as your server retries normally. Greylisting only delays the first message from a new sender.

All 191 bounce codes