Voicestream used to allow you to send a text message to any internet e-mail address but first putting the e-mail address in as the first part of the SMS followed by a space for the subject and a # to separate the body from the subject. Then you would send the SMS to 500. So in a sense it was e-mail via SMS.
T-Mobile still supports this (Voicestream is now T-Mobile) but they don't seem to have it clearly documented on their site, probably seeing as they are pushing their GPRS capability and want you to subscribe for that.
You can create an address book of short cuts for email addresses because you are still limited by SMS message size, so you probably don't want to have to type a long e-mail address.
With the Nokia 6310i, the send e-mail option in Message just automates the above. So for T-Mobile I would enter the e-mail address at the first prompt (E-mail address

, the subject at the second prompt (Subject

and then the body of the message in the standard SMS editing window which pops up next. Then select Options, Send e-mail and enter 500 at the E-mail server number: prompt. I haven't played with this enough to know if it support the newer long (i.e., split) SMS standard as I seldom want to type that much into my phone!
You should also be able to do this with most SMS programs on PocketPC, just set 500 as the number to send the SMS too and follow the format above for addressing and the subject and body.
I don't know if ATTWS support this same SMS mechanism for sending e-mails or if they do what number they use for the e-mail server. I don't use it very frequently but it's great for that infrequent occasion when you really need and don't have access to a computer, and it works great if you don't have your PPC either.
Hope this helps.
Cheers,
ajf