> Any idea what this is?  No such user on the list.  Spammer??
>
>
>   5 04/22 04/22 [log in to unmask]
>                 Last error: 5.2.0 x-unix; can't create user output file

Two possibilities:

(a) The address is a forwarded or remapped one for one of your subscribers.

(b) The address was the target of a virus email, which had picked up your
list address and used it as a "spoofed" sender address.

Given that the total errors count is 5, (a) looks more likely; the usual
result in case (b) is a single error, because a virus doesn't normally pick
the same combination of sender and receiver again.

This type of error may indicate a transient condition; all the errors are
reported for yesterday, and it's likely the UNIX system in question is
messed up and needed administrative attention, after which the errors will
stop (or, by now, would have stopped).

Hal Keen