Re: Previous poster wants message combining by sender. Another voice (me) prefers subject. Someone else will ask for date. I think we all know where this is going.
Last night, @StR had the foresight for the "convergence point" where this is going. I agree and have put it on my to-do.
Re: final word
As this new setting shows, there ain't no such thing as a "final word". For now, I did implement what I wanted.
Use cases:
1) repeated automated messages (forums, notifications) 2) mail apps which don't set the headers that are supposed to link responses to the originals.
Both are quite valid in my mind and it's useful to cover them.
Re: sent messages be grouped in one conversation by SENDER because you'll get only one conversation in your sent folder
This however still needs to be addressed, I consider it a bug fix in this new "sender + subject" code.
Re: "clustering" feature
Looks pretty close, if all the individual messages are From the same address and have same subject, which is quite plausible (e.g. "Daily deals" from "daily@deals.com").