freebsd-ports/mail/dkim-milter/pkg-message.postfix

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In order to run this port, please add the following lines to
${PREFIX}/etc/mail/main.cf:

smtpd_milters = unix:/var/run/milterdkim/filter

And to run the milter from startup, add milterdkim_enable="YES" in
your /etc/rc.conf.
Extra options can be found in startup script.

Note: milter sockets must be accessible from postfix/smtpd;
  using inet sockets might be preferred.

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