BW> Just to double-check, by "e-mail", you mean a message (OK, e-mail then)
 BW> in a Base Type: Local with Private: Yes? Or are there some other things
 BW> that signify an e-mail?
 BW> 

No, I mean the e-mail base. Its literally called "E-mail" in the fresh installation for the past 26 years. :)

You can't miss it as its base 1 and the configuration won't let you remove it! :) 

The e-mail base is accessed by using MW and MC menu commands, so to send email you'd use MW and to check it you'd use MC.

 BW> Simply post two e-mail messages to yourself in a Base Type: Local,
 BW> Private: Yes area. Read the second one, but not the first one. And call
 BW> MC /UNREAD; for me it displays a correct prompt #127, but no entries in
 BW> the list (prompt #126):

I did test it pretty heavily before I responded to say I couldn't reproduce it so there has to be more to it.  I did only test in Windows though.

MC does not scan message bases, so the steps you've given above would not work.

In Mystic each user has a private mailbox, and MC is used to check the messages in their private mailbox.  MW is used to send a message to another user's private mailbox.  In the 2000s Mystic expanded on this to integrate with Internet e-mail and SMTP/POP3 protocols.  This is separate from message bases but it seems you're trying to make it a message base.

MC does not open or scan a message base, only looks at data in the users private mailbox (base 1).

If you've somehow found a way to delete or change the e-mail base (which the configuration should prevent you from doing) then you're going to have problems.  You'd probably have to externally manipulate or delete data files that Mystic ships with for that to happen, so its likely something else is going on.

If you've configured a message base to write to or overwrite the mailbox's data files, then that could cause a problem.  I don't know what the problem could be, but its likely going to be some one off configuration thing or something obscure.  I am not able to reproduce it and the MC command has been in Mystic since the mid 1990s.  It seems unlikely that something that runs almost every time someone logs in was not working at all for that long and no one noticed...

There must be more to it than that.  Have you tried the same test with a fresh install?

... This virus requires Microsoft Windows 3.x

--- Mystic BBS v1.12 A47 2021/07/30 (Windows/64)