Roi , Here are some Ideas .Actually this is how I tested .
a) Configure any test domain on SMG
b) You should have two Servers other than SMG . One as your Mail Server ( Use Mail Enable or any other free Mail Server) and use other as SMTP generator (external or internal ) .For Sending ,Use any portable email client
c) Make some test Policies and send email
Scenario will be like this .
SMTP Generator software ----> SMG ---> Mail Server ( Test for incoming)
d)To test for Outgoing , you have to allow this SMG to be able to send emails to Internet .Once done , On your existing Email Gateway ,make a rule to forward email for a certain recipient to your test SMG
For a real environment like test , You can get a new MX Record with Lowest Priority for your existing domain Or get a new domain and new MX Record and route traffic to it .
SMG is a very good appliance and SMTP Routing is quite easy to configure . let me know if you need any help and if this post helps . please mark it as solution .