Hi.  I use VM as my mail reader.  I have it set up to FCC messages to
an mbox in ~/mail/Outbound, by setting mail-archive-file-name.

When Emacs writes a message to this file, it seems to prepend a blank
line, like this:

$ head -3 mail/Outbound

From ian Wed May 28 18:17:52 +0100 2025
Date: Wed, 28 May 2025 18:17:52 +0100
$

In previous Debian releases that I used, this all worked.  Nowadays it
doesn't.  Instead, "get new mail" in VM produces an error buffer
called `*output of movemail /home/ian/mail/Outbound
/home/ian/mail/INBOX.crash*` containing this:

   movemail: could not create mailbox `/home/ian/mail/Outbound': Requested item not found

   movemail exited with code 1

It's possible that in previous releases I was using a version of
movemail that came with Emacs.

Thanks,
Ian.