post can't send mail, but I'm using a very non-standard mailer. Going to
convert to postfix and will let you know if there are problems.
My general feeling about THAT is you should configure post to talk to
a preferred SMTP server, which is what every other MUA in existence
expects you to do (that SMTP server could live on localhost if you
really want to do that). I will acknowledge that not everyone agrees
with me on that.
The "standard" way post expects a local mailer to work is that it can
run it with some switches so it can speak SMTP to the mailer process. You
can run it in a mode where it just sends the message to to the mailer
process, at the cost of reduced functionality.
--Ken