Of course this is possible, as "From" address has nothing to do with your authentication with SMTP server.
Please see this code sample:
http://codesnippets.joyent.com/posts/show/1362[
^].
Moreover, formally, it does not matter where you are "getting mail". This is a different service which can be implemented by the same software but can have different URL, authentication; and a different protocol is used (what is it in your case — POP3? — does not really matter…).
—SA