SMTP codes
SMTP 450 Mailbox unavailable (temporary)
The mailbox is temporarily unavailable: busy, locked, or the server is greylisting you. Temporary.
- Class
- 4xx · Temporary failure
- Retried?
- Yes, the sending server retries
- Defined in
- RFC 5321
What to do
- Wait: greylisting accepts the retry in 5–30 minutes.
- Make sure your server retries from the same IP and with the same sender.
How it appears in a bounce
450 Mailbox unavailable (temporary)
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.