You have one of the following scenarios:
1. You have one router which interconnects all subnets, and another router to connect to Internet. Your default gateway on the exchange server set to "internet" router.
Problem: Your exchange server does not know how to access another subnets, it sends this traffic to default gateway, and your default gateway does not know where to send traffic as well.
Solution: You should either add correct routing to your exchange server for all remote subnets, or add routing to your "internet router" so it will forward traffic to appropriate router.
2. You have one router or layer 3 switch, which interconnects all subnets, and connects to the Internet. And this router has access list(s), which prevent your connectivity. As an variant you may have firewall instead of router.
If you have more questions, just e-mail me.
Mikshael Shavrov
MCSE W2K, Security+, CCNP, CCDP, CCSP, etc...