Your terminology is slightly wrong here. Postfix does not send outgoing mail on port 587 - it accepts mail on 587. This mail is then relayed to an MTA on port 25.
Anyway, that aside, I believe the answer to your problem is a slight master.cf over-sight. Please make sure you have this line in $postfix/master.cf
587 inet n - - - - smtpd
That should do the trick.
Christopher J. Buckley