SMTP code 450 is a temporary failure returned in response to RCPT TO or MAIL FROM when the specified mailbox cannot be accessed at this time. The mailbox exists but is currently locked, busy, or subject to a policy restriction that prevents immediate processing. This is different from 550 (permanent rejection) — 450 explicitly tells the sending server to queue the message and retry later. Common scenarios include mailbox migration in progress, temporary policy blocks (greylisting), or backend directory servers being momentarily unreachable.
Greylisting is an anti-spam technique that temporarily rejects the first delivery attempt from an unknown triplet (sending IP, sender address, recipient address) with a 450. Legitimate MTAs retry after a delay and succeed; spambots typically do not. First-time senders will always experience a delay of 5-30 minutes.
The mailbox is currently being accessed by another delivery process, an IMAP/POP3 session performing maintenance, or a backup operation. The server returns 450 until the lock is released. This is common with mbox-format mailboxes that use file-level locking.
The mail server needs to query an LDAP or Active Directory server to verify the recipient, but the directory is not responding. Rather than permanently rejecting the message, the server returns 450 to trigger a retry after the directory recovers.
450 is a temporary failure. Most MTAs will automatically retry on an exponential backoff schedule. If using manual SMTP, wait at least 15 minutes and try again. For greylisting, the second attempt almost always succeeds.
While 450 is temporary, double-check the recipient address for typos. Some servers use 450 for addresses that are in a pending state (e.g., recently created, migration in progress).
If every first attempt to a domain returns 450 and succeeds on retry, greylisting is likely active. This is normal behavior. Contact the recipient's mail administrator if the delay is problematic.
Ensure you are connecting to the correct mail server. If MX records recently changed, you may be hitting a server that no longer hosts the mailbox.
dig MX recipient-domain.com +shortCheck DNS Records
The SMTP server is temporarily unavailable, closing the connection. The sending server should retry later.
The server encountered an internal error while processing the command. The message should be retried later.
The recipient's mailbox does not exist, or the server has permanently rejected the message due to policy.