I do run Postfix (coming from slackbuilds.org) on my Slackware 13.37 as my IQ is well under sea level
You don't have to remove any package. Just check that /etc/rc.d/sendmail be not executable, which hopefully should be the case.
And once installed, chmod +x /etc/rc.d/rc.postfix then after configuration "/etc/rc.d/rc.postfix start" (which will be done automatically at next startup). Simple, isn't it ?
PS As surprising as it can seem, configuration files will be located in /etc/postfix.
Comme dit le proverbe "si Didier peut le faire, Kikinovak aussi".