[Exim] another kind of filter

I have a client that has is a ISP.

He wants that everytime a user sends an e-mail from the ISP, every message
should have a footer like:

"Come to our ISP, and so on...."

Tecnically:

We have all clients(Windows, Linux, Mac, etc...) that uses the exim mail
server running on the server. The clients uses SMTP to send e-mail from theirs
machines to the outside world:

Client1 -\
          \+------+
Client2 ---| EXIM |---> FILTER ----> OUTSITE WORLD
          /+------+
Client3 -/

(Sorry, I'm really bad at ASCII draws)

How can I do it using Exim? Is it possible?


Thanks,

Diego Lages