No restrictions that I'm aware of. The BB device is meant to only communicate with BB servers (for the bundled apps), whether it be a BES (Enterprise) or BIS (Internet, aka persona). What happens is you configure an account on the BIS, set up that account to fetch mail from a POP/IMAP source, then the BIS will mediate communications between your handheld and your mail server.
Similarly, when you browse a web site (with the built-in browser), your handheld is communicating with the BES/BIS, which proxies the communication to the web site.
Assuming Orange UK
, this link should work. You can register for a BIS account, then set up your mail account on the BIS. There is, at this time, no cost for setting up or using the BIS. It supports up to ten mail accounts, is tightly integrated with GMail and Yahoo, will do IMAP and POP3, as well as OWA. For more info, scan the boards and you'll find a wealth of information.