As with most large ISP's, my ISP filters port 25 (SMTP). As a web hosting provider, I felt that my customers should be able to use my email server to send email, as well as receive it. It's kind of a value added feature
So, here's how I did it:
Open /etc/postfix/master.cf and add the following line:
587 inet n - n - - smtpd
This is for postfix only. Restart postfix and your done. This doesn't really have anything to do with ISPConfig, however maybe it should be an option...?
EDIT: I forgot to tell you, SMTP will listen on ports 25 and 587