SMTP
450 Mailbox Unavailable
The requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.
When You See This Code
When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
How to Fix
Retry delivery after a few minutes. If using greylisting, the second attempt will typically succeed. For persistent failures, check if the recipient's mailbox is full or disabled.