What does 550 mean?

SMTP 550 is the most common hard bounce code:

What it means:

The requested mailbox is not available. The most common cause is the user does not exist. This is a permanent failure.

Common causes:

User does not exist: Address was never valid or was deleted. Account disabled: User's account deactivated. Mailbox not allowed: Policy prevents delivery. Rejection due to policy: Content or sender blocked.

Typical message examples:

"550 User not found" "550 Mailbox not available" "550 No such user" "550 5.1.1 Recipient rejected"

Response:

Suppress this address immediately. Do not retry. Remove from active lists.

550 is a closed mailbox. The intended recipient is not there.