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.

This package conflicts with the stock postfix (or sendmail) package
included in Slackware and overwrites some of its files,
so remove the postfix (or sendmail) package before installing opensmtpd

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

With OpenSMTPD 7.8.0 and newer versions with the default "mbox" in the
/etc/opensmtpd/smtpd.conf then the permissions of /var/spool/mail 
may not be the Slackware default of 1777 and must be changed to 755
 instead:

  chmod 755 /var/spool/mail

As an alternative to "mbox" one can use "maildir" as this stores the
 messages in each users home directory instead of /var/spool/mail.
D. J. Bernstein answers the question, "Why should I use maildir?" here:

  https://cr.yp.to/proto/maildir.html

You may want to modify rc.local so that OpenSMTPD
will start on boot something like this:

  # start opensmtpd a replacement for sendmail and postfix
  if [ -x /etc/rc.d/rc.opensmtpd ]; then
        . /etc/rc.d/rc.opensmtpd start
  fi

Optional Dependency: LibreSSL

To use the LibreSSL runtime libraries
   First install LibreSSL 
    then build   OpenSMTPD

For hints on using alpine with OpenSMTPD see the README.alpine file