What are common 5xx codes and their meanings? (e.g., 550, 553, 554)

Common 5xx permanent failure codes:

550 Mailbox not found: User does not exist at this domain. Suppress this address.

551 User not local: Server does not handle mail for this address. Address may be incorrect.

552 Message size exceeded: Message too large for recipient. Reduce size or contact recipient via other means.

553 Mailbox name not allowed: Address syntax invalid or policy rejection. Check address format.

554 Transaction failed: Generic permanent failure. Read accompanying message for details. Often indicates policy rejection, spam filtering, or blocklisting.

Response to 5xx codes:

Do not retry. Suppress address for recipient-specific errors. Investigate and fix for sender-side issues.

5xx codes close the door. Accept the rejection and respond appropriately.