If you're talking about the old FidoNet style of E-mailing, it was usually a username for the BBS system that would be receiving the E-mail and then the Fidonet address, which was a series of numbers like 1:126.32.1 (I may be off a little here). The FidoNet address scheme went something like:
Country:Region.Network.BBS
Anyway, we used to call sending private mail between users of different BBS's "NetMail" since teh BBS usually had to dial the receiving BBS directly to deliver its netmail OR pass the netmail through the network. The problem was, not all BBS's allowed passing NetMail, and many did not allow their users to send Netmail. On the BBS I ran, I allowed users that had donated to the BBS to send netmail, and my system actually did connect to the receiving BBS directly to deliver it.
For non-private mail, there was many different E-Mail systems and they worked a lot like a mailing list works today. Folks could read mail on the BBS, or use a utility like BlueWave to download all the mail and read it off line, then upload their replies when they were done.
This really brings back some memories.
George