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, smtpf, smtpl, and smtpq users on the system
for privilege separation - something like this should suffice:
useradd -u 270 -g 0 -r -s /bin/false -d /var/empty smtpd
useradd -u 271 -g 0 -r -s /bin/false -d /var/empty smtpf
useradd -u 272 -g 0 -r -s /bin/false -d /var/empty smtpl
useradd -u 273 -g 0 -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.