What does 450 mean?

SMTP 450 indicates temporary mailbox unavailability:

What it means:

The specific mailbox cannot receive mail right now. The problem is with the recipient's mailbox, not your message. Retry is appropriate.

Common causes:

Greylisting: Intentional delay for unknown senders. Mailbox locked: User performing operations. Policy evaluation: Server deciding whether to accept. Temporary suspension: Account issues being resolved.

Typical message examples:

"450 Requested mail action not taken: mailbox unavailable" "450 Try again later" "450 Greylisted"

Response:

Retry after delay. Most greylisting clears after 5-15 minutes. Do not suppress on first occurrence.

450 often indicates greylisting. Patience usually resolves it.