To enable postfix rc.d startup script please add postfix_enable="YES" in
your rc.conf

If you not need sendmail anymore, please add in your rc.conf:

sendmail_enable="NO"
sendmail_submit_enable="NO"
sendmail_outbound_enable="NO"
sendmail_msp_queue_enable="NO"

And you can disable some sendmail specific daily maintenance routines in your
/etc/periodic.conf file:

daily_clean_hoststat_enable="NO"
daily_status_mail_rejects_enable="NO"
daily_status_include_submit_mailq="NO"
daily_submit_queuerun="NO"

If you are using SASL, you need to make sure that postfix has access to read
the sasldb file.  This is accomplished by adding postfix to group mail and
making the /usr/local/etc/sasldb* file(s) readable by group mail (this should
be the default for new installs).