Why Mail-Followup-To header for a non-list address?

[raf]

Hi,

Someone recently emailed me. Technically it was a reply to an
old email of mine. Since then, a few emails have gone back and
forth between us. All of my outgoing mails to this one address
have had a Mail-Followup-To header added. I have no idea why.
The address isn't mentioned in any "subscribe" or "lists" commands.

I don't have any "lists" commands. I do have a "subscribe" command
which refers to mailing lists by their aliases. One of the aliases
is "debian" and the email address in question does contain "+debian"
but that shouldn't matter.

I've fixed it with a send-hook that does "set followup_to = no"
for that address, but I don't understand why I needed to.

Can anyone think what I might have done to cause this?

Linux ook 6.1.0-11-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.38-4 (2023-08-08) x86_64 GNU/Linux

mutt-2.2.9-1+b1

cheers,
raf