Exit Relays can be set to allow traffic to exit the Tor network only over certain ports. Vidalia even makes this super-simple by giving options like "Normal, unencrypted Web browsing" or "IRC clients and servers".
This answer notes that:
Tor chooses an exit relay which actually allows you to exit the Tor network. Some only allow web traffic (port 80) which is not useful when someone wants to send emails.
What happens when a user is passing traffic over multiple ports? It wouldn't be uncommon to receive email over IMAP whilst web browsing, or even to visit a page serving a mix of HTTP & HTTPS content. Would Tor have to find a single Exit Relay which allows all ports needed, or is it possible that traffic might exit a different node for each port?