Messaging Gateway

How could I use the evaluation version of Messaging Gateway in order to try it without touching my infrastructure?

I mean, is there a way in order to use MG with some "external" free mail server or so on?
I'd like to evaluate functionality without touching production environment.

I would install the SMG in an isolated testing enviornment for the durantion of your evaluation.  I have not heard of any "external" free mail servers that could be used for evaluating the SMG.

I think you can install sendmail as a mail server, however unless you point your production domain mx to the SMG you will not be able to see all the benefits.