New comment by Duncaen on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/218#issuecomment-974629671

Comment:
This happens if there is a mail with `In-Reply-To` header to a message id that is not part of the mails passed to `mthread`, so instead of silently ignoring this, it prints the message id and uses that as parent.
Sequences that are not threaded don't look at the `In-Reply-To` header so they don't add those missing message ids.

This could have been either someone mailing the person replying to the non-existing message-id directly instead of the list who then replies back to the mailing list.
Another possibility is that the message ends up in spam filter/folder and is not part of the sequence passed to `mthread`.
Some automatic mails, like from github issues will just put a non-existing message id into `In-Reply-To` to group mails about issue comments into a thread which are not actually threaded.