(1) The stock sendmail.cf will only listen on the loopback interface.
(2) The stock submit.cf specifally connects to "localhost." which
should be less susceptible to being confused or looking confused.
(3) The smtp listener starts by default, if needed. The setting in
/etc/default/rc.conf is still "no", but rc.d/sendmail detects the
default setting and will change it to yes if need is determined.
Need is defined as "nothing else seems to have been changed about the
mail configuration but we'd like locally originated and locally
destined mail to be delivered". If you change, eg, mailer.conf to
point to postfix or some other MTA, sendmail will not start.
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