Here's what I found using NNTP with Mystic BBS:
Mystic's NNTP server at default port 119 is made to act like Usenet.
In Alpha 31 it's working fine with Thunderbird and other Newsreaders and
sends to All as per the Usenet standard. To use it with Mystic you need
to setup the Message Base (Configuration, Editors, B Message Base Editor)
to use a Newsgroup name like Usenet does, so best using a 6 letter all
capital name like BBSNAME.MESSAGEBASE. Even local groups can be setup
this way to be read/written with a newsreader.
Using Thunderbird to create an NNTP account generally under Local Folders, Create a new account: Newsgroup. Put in your BBS Handle for Your Name, and likely a bogus Email Address of BBSHandle@none just so Thunderbird will
take it (not to really be used). Next, Newsgroup Server is the BBS telnet address, Next, Account name likely use the BBS Name, Next, Finish. Then
Select account, right click, and choose Subscribe. After some time a list
of areas to subscribe to should appear and you'll have to drill down to
the full name to click the check box and subscribe to each area you want.
When this is done it will make a list under 'BBS Name' of areas that will
show F.M.BBS for FSXNet.Mystic.BBS for example. After that click on the
area name and either right click and use Get Messages or select the area
name and click 'Get Messages' at the top of the screen. It will ask for
the number of message headers to load with 500 as default.
The only other thing to note is to use "Followup to Newsgroup" to reply.

--- Mystic BBS v1.12 A31 (Raspberry Pi)
* Origin: Mystic Pi BBS bcw142.zapto.org (21:1/145)