try recreating your database access file /etc/mail/accass.db, with the command:
"makemap hash /etc/mail/access.db < /etc/mail/access"
If you're on stock install of slack 9.0, you should be able to send (and receive) email on your LAN, as well as send only on your WAN, after that.
I spent most of my day just serching the net for this ... I know your sendmail associated pain