i have configuring postfix as relay mail server, and in this time have running well. so can ask implemented with this condition ?
ISP Mail ----> Postfix Relay ----> Exchange.
If you have a list of all email addresses on your relay server and forward emails after processing to other email it should be possible. But for better answer we need more info how your relay server works and what it does exactly.
I run a fido gateway based on fidogate and have fido points here. I got it to work, that they can use ASK too. Is is not easy, but it works :-)