Lars wrote:
>  I'm unable to reproduce this in Emacs 28, but I'm not quite sure I'm
>  testing it correctly.  Are you still seeing this issue in recent Emacs
>  versions?

    Yes, I can reproduce this in version 28.1.

    I'm attaching a *mbox* file (this is the format used by rmail),
lightly edited from one of my older rmail files to hide email addresses.
It has two messages, the first with zero blank lines at the end and last
with 2 blank lines at the end.  If you find file it then do M-x
rmail-mode and look at the first message, you will see that it has an
extra blank line at the end when displayed.  The last one doesn't.  (go
to the end of message with esc > or turn on line numbering to see where
the displayed message ends.)

    If you look at the rmail file before doing rmail-mode, you can see
the underlying formatting.  You will see a blank line between the first
and second message; this is part of the mbox formatting, not a part of
either message.

from https://en.wikipedia.org/wiki/Mbox
    The mbox format uses a single blank line followed by the string 'From '
    (with a space) to delimit messages; this can create ambiguities if a
    message contains the same sequence in the message text.

(The original reproduction instructions probably no longer work because
it's actually quite hard to send a message that's not using MIME these
days.)

    To add insult to injury, if you attempt to forward the message, Rmail
will include the spurious blank line in the message it forwards.  I hit
'f' for this in emacs28.1 -q and get a # part with the blank line before
the closing tag.

- Mark
PS, I have gzipped the mbox file to prevent intermediate mailers from
corrupting it.