I was having success using the Connector class with URLs like "socket://ip-addrort" on all of the Sun device emulators, and the Nokia Series 40 emulator. Of course, on the 7210 emulator, it tells me that I can't access a system class. I've tried tricking it, by subclassing the systems classes in my own class, or creating my own classes in a system package (because they obviously can access system classes), but then the phone says "can't create class in system package". So far I'm out-smarted.
I had assumed that sockets just weren't possible on such devices, and have started to build an HTTP based solution instead. But today I see a product (http://www.reqwireless.com/emailviewer.html) that apparently talks POP3, IMAP and SMTP on Java phones, including the 7210 (indeed, the whole Nokia range). I guess they might actually be diverting requests via their own webserver, but I kind of suspect that they might know something I don't.
If they do, they have a competitive advantage. Which doesn't seem very fair. Won't Nokia tell us all how to do this!!!???!!