You can find the Postfix web site at http://www.postfix.org/.

1. Stop Sendmail prior to installing Postfix. The Postfix installation will modify the startup/shutdown scripts, making it more difficult to properly stop Sendmail if it is still running at that point.

   /etc/rc.d/sendmail stop
   

2. Install Postfix from the ports tree and configure it to start at boot with the following commands:

   portinstall -m BATCH=yes mail/postfix
   ed
   a
   #
   # Execute the Postfix sendmail program, named /usr/local/sbin/sendmail
   #
   sendmail      /usr/local/sbin/sendmail
   send-mail     /usr/local/sbin/sendmail
   mailq         /usr/local/sbin/sendmail
   newaliases    /usr/local/sbin/sendmail
   .
   w /etc/mail/mailer.conf
   q
   ed /etc/rc.conf
   a
   sendmail_enable="YES"
   sendmail_flags="-bd"
   sendmail_pidfile="/var/spool/postfix/pid/master.pid"
   sendmail_procname="/usr/local/libexec/postfix/master"
   sendmail_outbound_enable="NO"
   sendmail_submit_enable="NO"
   sendmail_msp_queue_enable="NO"
   .
   wq
   

3. Disable FreeBSD's default Sendmail-specific mail maintenance routines with the following commands:

   ed
   a
   daily_clean_hoststat_enable="NO"
   daily_status_mail_rejects_enable="NO"
   daily_status_include_submit_mailq="NO"
   daily_submit_queuerun="NO"
   .
   w /etc/periodic.conf
   q
   

4. Reconfigure Postfix to listen only on the loopback interface, mimicing sendmail's default listener configuration, with the following command:

   sed -i '' -e 's/^smtp *inet/localhost:smtp inet/' /usr/local/etc/postfix/master.cf
   

5. Optional: In their default configurations, both sendmail and Postfix will try to deliver messages directly to their destinations. If desired, you can configure Postfix to relay mail through a proxy with the relayhost setting. Reconfigure Postfix to relay to the mail exchangers of the computer's DNS domain (as listed in that domain's MX records) with the following commands:

   ed /usr/local/etc/postfix/main.cf
   a
   relayhost = $mydomain
   .
   wq
   

   If you prefer to relay through a specific host, use these commands instead (replacing mail.example.com with the host name or IP address of your relay):

   ed /usr/local/etc/postfix/main.cf
   a
   relayhost = [mail.example.com]
   .
   wq
   

6. Start Postfix with the following command:

   /etc/rc.d/sendmail start