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.
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