JH> We'll need to be able to configure the "send from" email address in
 JH> Mystic for this to be able to work.

I have this "done already but I just haven't had time to test it.  There will be a resetmail.ini that can be defined by theme or globally in the data folder that will override the defaults.  It looks like this:

; Any text after [Text] is used as the e-mail content
;
; @CODE@ will be replaced in the [Text] section with the code required
; for password reset.  Traditional MCI codes are available for from and
; subject lines but not in Text unless enabled by setting textmci to true

[General]
    from_name = |BN
    from_addr = sysop@|DN
    subject   = E-mail validation code for |BN
    textmci   = false

[Text]
Enter the following code when prompted by the BBS to validate e-mail:
@CODE@

... Help! I can't find the "ANY" key.

--- Mystic BBS v1.12 A48 2022/06/03 (Windows/32)