Antispam with Astaro

Hi all
I have a little question.
A customer have a normal lan with a router cisco,20/30 pc client and a server with 9 VM.
In this VM are a BES and a mail server and I think that all is in the same lan.
Question:Is a good idea to put a UTM9 with email security in the server and use it only to antispam ?
Maybe add other subscriptions later ....
The customer would like make less change possible at him lan.

Thank you very much

Luca