SMTP Error 450 - Mailbox unavailable (temporary). | Warmup Inbox

The short answer

SMTP 450 is a temporary rejection: "mailbox unavailable." The receiving server deferred your message, usually for greylisting, rate limiting, or a busy or over-quota mailbox. The message is not lost. Retry with backoff instead of treating it as a bounce.

Quick fix

Retry later — greylisting usually clears on the second attempt.

4 - Temporary. Safe to retry. The message is not lost and most servers retry automatically.

What it means

450 is defined in RFC 5321 as "Requested mail action not taken: mailbox unavailable." The first digit is what matters. A reply starting with 4 is a transient failure: the command did not go through, but the condition is temporary and the same message can be tried again. That is what separates 450 from the permanent 550. A 450 usually carries an enhanced code that narrows the cause: 4.2.2 for a full mailbox, 4.3.2 for a server not accepting mail, 4.7.1 for a policy hold. Some receivers reuse 4.2.1 for rate limiting (Gmail returns "450 4.2.1 ... receiving mail too quickly"), so read the full text the server sends.

Common causes

Greylisting
Many servers reject the first attempt from an unfamiliar sender with a 450 on purpose, expecting a real mail server to retry. Spam bots usually do not; legitimate mailers do.
Rate limiting
You are sending faster than the receiver allows per IP or per recipient, often shown as 4.2.1.
Busy or over-quota mailbox
The recipient mailbox is full or locked during maintenance (4.2.2).
Policy or reputation hold
The receiver is deferring your mail while it evaluates reputation, or is under load (4.7.1, 4.3.2).

How to fix it

Retry with backoff first
For greylisting, a single retry a few minutes later usually delivers. Most mail servers retry automatically over hours or days. Do not delete or rewrite the message on the first 450.
Read the enhanced code and text
4.2.1 means slow down, 4.2.2 means the recipient is over quota (their problem, retry later), 4.7.1 means a policy or reputation hold.
Reduce your send rate if throttled
Spread volume out, respect per-domain limits, and ramp new sending IPs up gradually.
Authenticate to bypass greylisting
Publish and align SPF, DKIM, and DMARC. Authenticated, reputable senders are often skipped past greylisting entirely.
Watch for IP rotation on your ESP
Large providers rotate outbound IPs. If a retry leaves from a different IP, some greylisting sees a new sender and defers again. A consistent IP or a provider that handles retries internally avoids this loop.

Paired code

A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:

X.2.1 Mailbox disabled

Common questions

Is 450 permanent or temporary?
Temporary. Any code starting with 4 is a transient failure and should be retried. The permanent counterpart is 550.
What is the difference between 450 and 550?
450 is a temporary defer; the message may deliver on retry. 550 is a permanent rejection; the server will not change its answer.
Why do I get 450 only on the first send to a new recipient?
Greylisting. The receiver defers unfamiliar senders once and accepts them on the retry.

All SMTP error codes

Most rejections come back to sender reputation.
Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.