What do 5xx SMTP error codes mean? (Permanent failures)

5xx SMTP codes indicate permanent, fatal conditions:

The signal:

"I will not accept this message. Do not retry." The failure is definitive. Retrying would be wasteful.

Common 5xx codes:

550: Mailbox not found or unavailable. 551: User not local. 552: Message exceeds size limit. 553: Mailbox name not allowed. 554: Transaction failed (generic).

Sender response:

Do not retry this message. Suppress the address (for mailbox-specific errors). Investigate and resolve (for sender-side issues).

Important distinction:

Some 5xx codes indicate recipient problems (suppress address). Others indicate sender problems (fix your configuration). Read the full error message for context.

5xx is a firm "no." Respect it and move on.