[Exim] How would you make ?

Hello !

I manage a MTA (exim of course) mutualized for my customers.
On this server they (my customers) can define their virtual domains
and accounts user themself. And then they can send their mails 'freely'.
All of them are our customers and know by our services.
"Voila" it was the description and now my 'problem'... :)
This MTA delivers, on average, more than 200000 mails per week, and
a large part of these mails are of my customers. In this part there's
- sometimes - another part of spam..... :[
I would like to control that but it's very difficult because i cannot
control the contents or topics... Sometimes it's Adult, sometimes it's
Games, sometimes it's Services... So it's not easy to know what is spam
or not.
Then, my idea is "Perhaps can i restrict/limit the CC: BC: or the number
of mails sent by a sender." etc...

What do you think ? What is your solutions ?

Thank you.

Lilian.