Any APRS client that has messaging capability also has the capability to send internet e-mail. Receiving is another issue however, due to FCC regulations. An internet or RF-connected APRS user can send messages to any other APRS user though.
To send an internet e-mail, one method is as follows:
To: EMAIL Message: email@example.com Here is my text message.
A server in New Jersey will pick up that message, send it to the internet e-mail address listed at the beginning of the message text, and you should get a response shortly back from the e-mail server letting you know if the message was delivered.
There are other e-mail servers as well, perhaps people will populate this page with more data so that they can all be properly documented in one place.