Can you send yourself email from inside the network? For instance, when you set up a new user in ispconfig and give them a user name / email / etc, can you then (on the server) run these commands?
sendmail -v firstname.lastname@example.org
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ctrl+d (to send)
using "-v" will send the mail to your root account with verbose output.
Also this may seem obvious, but do you have port 25 opened and being forwarded to that box from your router / firewall?
also, can you do this:
telnet localhost 25
also tail /var/log/mail.log or /var/log/maillog ... something like that