Fixing "550 5.1.1 User unknown / no such user"

550 5.1.1 is a recipient-side rejection: the receiving server accepted the connection but has no such mailbox. The problem is the address, not your sending setup.

What it means

The local part before the @ does not exist at that domain. It is a hard bounce — the server is certain, so retrying will not help.

Why it happens

A typo in the address, a mailbox that was deleted, or an old address from a stale list. If many addresses at one domain fail, the domain’s mail routing may have changed.

Diagnose it

Confirm the recipient domain still has valid MX records with the MX tool — if the MX is fine, the address itself is simply wrong. If the whole domain fails, see the no-MX guide.

Fix it

1. Double-check the exact spelling of the address with the recipient.

2. Remove hard-bounced addresses from your list immediately — repeatedly mailing dead addresses hurts sender reputation.

3. If it is your own domain rejecting valid users, fix the mailbox/alias or the inbound routing table.