I am not pretty sure if it is a bug or not implemented. I was playing with different settings and i failed to group mails by subject. Here how it looks what i mean in Thunderbird. http://storage6.static.itmages.ru/i/13/0722/h_1374511153_6196986_f7a10a2c40.png Reproducible: Always
This works perfectly, you need to configure the messages aggregation. Go to View->Message List->Aggregation and select 'Current Activity, Threaded'
I've tried every predefined view, but with no desirable effect.
Ok, I see the problem now. My mails are shown like in your screenshot only when replying mails start with "RE:". If they have the exact same subject they don't get grouped. Not sure if it's a bug, but it's definitely an interesting feature to have.
This bug has only been reported for versions before 4.14, which have been unsupported for at least two years now. Can anyone tell if this bug still present? If noone confirms this bug for a Framework-based version of kmail2 (version 5.0 or later, as part of KDE Applications 15.12 or later), it gets closed in about three months.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.0 aka 15.08), please open a new one unless it already exists. Thank you for all your input.