OpenSMTPD is a FREE implementation of the server-side SMTP protocol 
as defined by RFC 5321, with some additional standard extensions. 
It allows ordinary machines to exchange e-mails with other systems 
speaking the SMTP protocol.

You must have smtpd and smtpq users groups on the system 
for privilege separation - something like this should suffice: 
groupadd -g 270 smtpd
groupadd -g 271 smtpq
useradd -u 270 -g 270 -r -s /bin/false -d /var/empty smtpd
useradd -u 271 -g 271 -r -s /bin/false -d /var/empty smtpq

You will want to add /etc/rc.d/rc.opensmtpd to rc.local so that it 
will start on boot (or perhaps consider symlinking rc.sendmail to
rc.opensmtpd).

This package conflicts with the stock sendmail package included in
Slackware and overwrites some of its files if you do not remove it
before.