I believe both have a set of addresses and are fully bidirectional. We have general corp web surfing, light inbound web traffic for a few customers to specific web applications, and "production" FTP, it, lots of heavy FTP traffic going both directions.

We have a 3rd T1 that is usually email only on a different router.

I got bottlnecked on getting a file via FTP from a vendor. So I was given an account on the email server just to pull the file over the 3rd T1. But I would rather the other actually works! We have a bunch of "production" people who are supposed to be moving stuff around. I only got involved when my project was going to be late because the file was not showing up, and then I screamed loud enough to be given an account on the email server just for FTP. Not the norm!