ok so you have a VPN to site 2, which normal traffic flows through the secure line to 172.x.x.x
However, you want email to go out the 192 correct?
Are you connecting to email via POP?
If you're connecting directly to an exchange server, that means you'll have to leave open RPC ports on remote end... NOT a good suggestion.
Hmmm
If you type 'route print' from command line you can see which networks go out which gateway.
Theoretically you could edit that, but certainly not something you want to do for fun