slackbuilds/network/opensmtpd/README

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 and groups on the system for
privilege separation - something like this should suffice:
  groupadd -g 270 smtpd
  useradd -u 270 -g 270 -r -s /bin/false -d /var/empty smtpd
  groupadd -g 271 smtpq
  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, so remove the sendmail
package before installing opensmtpd.