Hi Tom,
The first thing you should know is that the D-STAR format does not support messaging directly from the radios. All D-STAR radios however, have a serial data port and support sending "low speed data" along with the voice traffic. It is best to think of a D-STAR radio as a modem, with about 900 bps through-put. With a PC connected at each end, you can chat, send messages and transfer files. If one station has an internet connection, you can send and receive plain text e-mails using Winlink or your personal internet e-mail account. The radios can operate simplex or through a repeater and gateway, as long as there is a path between the two radios. You should ask the repeater operator how they feel about data only traffic on their system as you will appear to other users as if you are transmitting and not saying anything. D-RATS does not have a nationwide network, like APRS, which lets you send short messages from the keypad of the radio.
For more information about D-RATS, download the Operating Guide and it's addendum, and the "D-RATS basic e-mail features" quick reference sheet from the files section.
As to non-Hams on amateur radio, consider the phone patch. You can contact non-Hams by phone, but the type of traffic is restricted. You can call home and tell them to order pizza for supper, but you can't call the pizza shop and order it yourself. So health and welfare e-mails are allowed.
I hope this answers some of your questions.
Patrick Connor