That is probably nothing you can change in ISPC. You will have to change the config of your smtp server. If you are running Postfix then edit /etc/postfix/master.cf and change the smtpd line to something like:
10025 inet n - n - - smtpd
to listen to port 10025. From your post I assume you have some spam filter that will filter the mail before handing it over to Postfix. That spamfilter will of course need to be configured to deliver the mail on port 10025 in the above example.
If you are running sendmail then someone else should answer your question
Personaly I would prefer to do it the other way around since Postfix has excellent features for using external mailfilters such as amavisd-new and the possibility to block mail from blacklisted hosts. But that's just me...