That message just says that your mail client is checking your mailbox using POP3.
Receiving a mail with an unknown recipient generates a log entry like this:
May 11 07:08:09 mail-gw sm-mta: q4BGPcqJ018609: <firstname.lastname@example.org>... User unknown
Again, are you sure
this is the only server involved in receiving mails for this domain? Does host -t mx domain.com
list the name of the Slackware server, and nothing else? And does this name resolve to the public IP address of that server?
Also, do you see the names of the other users in /etc/mail/aliases