Mail conversation half-works

To help us better diagnose and, on the basis of that, hopefully fix the issue, it would be good to understand what you mean by "half-works"

-Max

Some mails are grouped (by conversation), many others not.

I don't know why, it worked fine in yosemite.

Some mails with the same subject are grouped, other not. I don't know how mail app put a mail in a conversation group.

Right, I know what you mean now. You're not the only one highlighting this - a couple of other people started discussing it here:

https://forums.developer.apple.com/message/53486

I noticed too that Mail seems to be following a logic that is very strict about what constitutes the same Subject - even the "Re:", that often gets added to the beginning of the suject line by mail clients, causes them to be deemed unrelated. Previously Mail's conversation rules overlooked that I think...

-Max