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

Gmail has no mailbox with this address. It is the most common hard bounce there is.

Why it happens

A typo before the @.
The account was deleted. Google also deletes personal accounts that go unused for a long time.
At a company on Google Workspace: the person left, or the address was an alias that was removed.
A space or a dot at the end of the address, copied along by mistake.

Check the spelling. Note that in @gmail.com addresses dots do not matter (j.smith and jsmith are the same mailbox), so there a dot is never the reason.
If the spelling is right, remove the address. Do not send to it again.
For a whole list, the email typo checker finds mistyped domains such as gmial.com. It cannot tell whether a Gmail name exists; only a bounce like this one can.

550 5.2.1 means the account exists but is disabled. 550 5.7.1 means the address is fine and Gmail refused the message itself.

Yes. Remove the address after one bounce.

One does not. A mailing where many addresses bounce this way tells Gmail the list is old or bought.

Paste the whole message into the bounce code lookup. It reads the code and the words and tells you whose move it is.