There is a wonderful View->Sort option called "attract by subject" which is suitable for sorting unusual folders and types of messages to make a pseudo-threaded environment.

One such example is a web-based forum I use that Emails new messages to the users, creating a kind of pseudo-mailing list. They are not in "reply to" format, but will have identical subjects for each posting of the same forum thread.

The problem is that Claws doesn't have an option to REMEMBER or "turn on" this "attract by subject" feature on a per-folder basis.  It seems you can use it to temporarily sort/group and then as soon as you leave that folder and return to it, it goes right back to be un-attracted.