Non-user logins?

[Ken Wright]

My Dovecot issues continue.  Right now I see at least two issues: 
first, my logs consistently show non-users trying (and failing) to log
in, and I'm still unable to log in from my email client (Evolution or
Roundcube, either one).

I'll post about the second issue later; right now I wonder why I'm
getting so many non-users trying to log in.  Am I the subject of 
concerted hacking attacks, or is there something else going on?  Some
of the attempted logins are more-or-less random names claiming to be
@mydomain, but at least one is a username that's really on my server,
to wit:

Jan  7 22:52:01 grace dovecot: lmtp(776281): Error: lmtp-server: conn
unix:pid=776262,uid=117 [3]: rcpt www-data at mydomain.com: Failed to
lookup user www-data at mydomain.com: Internal error occurred. Refer to
server log for more information.

(Another quick question:  which server log should I check?)

So, if anyone can tell me what's going on with all these logins, I'd be
much obliged!


Ken