550 5.1.1: the mailbox does not exist

The mailbox does not exist on the receiving server. The domain is fine and your authentication is fine; the part before the @ matches no account there. Check for a typo, then treat it as permanent and remove the address from your list — retrying a hard bounce is what damages your sending reputation.

The error

550 5.1.1 The email account that you tried to reach does not exist.

550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table

What it means

The address does not exist on the receiving system. This is the single most common bounce there is.

Something about the message or the address you sent to needs changing.

Why it happens

A typo, a person who has left the organisation, or an address harvested from a form with no confirmation step. Continuing to send to addresses that return 5.1.1 is one of the fastest ways to damage sending reputation, because a high unknown-user rate is exactly what spam filters look for.

How to fix it

1. Check the address for a typo — transposed letters and wrong domains are most of these.
2. Remove it from your list. Do not retry: the answer will not change.
3. If you are getting many of these at once, your list needs cleaning before you send again.
4. Add confirmation to any form that collects addresses, so typos never enter the list.