you will need to find the sendmail.mc file that was used to configure sendmail to begin with.
then you edit it to have:
DAEMON_OPTIONS(`Port=smtps,Addr=xxx.xxx.xxx.xxx, Name=MSA-SSL, M=E')
the smtps is if you are using sasl or some other kind of secure connection
after it is edited then you need to create the sendmail.cf file, copy it over the current one, for slackware that is in /etc/mail
then restart sendmail
make sure you have the virtual IPs set to be created in a script that runs before sendmail starts, otherwise sendmail will fail because the IP doesn't exist