Tell me more... I'm a sysadmin who loves a challenge. What else do these guys do? I would love to read their end user license agreement. Probably says something like "WE READ YOUR EMAIL!"
Go to here
and see if your ip address always changes, if it does everytime then they are proxying you in a pool of addresses, if it changes everytime you reboot your dsl modem, that's just because it's dhcp. I would be very very surprised if they proxied everything. Plus you can't proxy SSL sessions without breaking them, so things like gmail should act normally if they are doing things they aren't meant to...
What make is the DSL modem?