When user all over the internet today send messages back to other users online instantly this is called electronic mail or so called "email".
To reference it with BBSing this was called Netmail, used within a network called a echo. Netmail was sent by one user within that particular echo to another user that was also part of the echo. So another words, both users would have to be on bulletin board systems that carried the same echo for them to send messages to each other.
If it was around the world, even next door as they say it wasn't there as fast as the internet but it was usually there the next day.
So someone in Wisconsin, USA could write a Netmail message to someone else in another country part of the same network and have it sent their way, it usually would arrive the next day. This was called Netmail, today called
email for the ones that want to hold on to what email is called today.