I use Mutt for emails. I have threads enabled and I can see that most of the emails are threaded in Mutt. So that is all good. But sometimes I reply to an email and the answer(from other person) to my replied email wont be part of any threads. The thing is that when I reply in Mutt (which I use Vim to edit them), the subject parts keeps getting longer and longer with many "Re"s. That is the case with those emails with missing threads.

I have:

set strict_threads="yes"
set sort="threads"
set edit_headers=yes

I am wondering if this has anything to do with Mutt or the person I am communicating over email.

Could this one be the culprit?

set metoo=yes

Any suggestions?