Hard bounce

Also called: Permanent bounce, Permanent failure.

Under RFC 5321, reply codes starting with 5 mean permanent failure. The most common hard bounce is 550 5.1.1, where the enhanced status code 5.1.1 (RFC 3463) means "bad destination mailbox address": no such user. Other hard bounces include domains that don't exist or accept no mail.

A hard bounce means stop. Microsoft's Outlook.com policy says that after a permanent failure the sender must not try to retransmit that message to that recipient, and Amazon SES bases its bounce rate on hard bounces. Every hard bounce should go straight onto your suppression list so no future campaign, client or export sends to it again.

Not every 5xx reply is about the address. Codes in the 5.7.x range are usually policy rejections of the sender, such as failed authentication. Outlook, for example, rejects non-compliant high-volume mail with 550 5.7.515. Those look like hard bounces in a dashboard but point to your setup, not your list.

For local business lists, hard bounces mostly come from guessed addresses, former staff mailboxes and closed businesses. Verifying shortly before sending removes most of them.

Example

An email to jsmith@acmedental.com comes back with 550 5.1.1 The email account that you tried to reach does not exist. The address is suppressed permanently.