Hi.

Threaded mails would break in case the "Long Thread" Of Mail is involving hotmail. They just seem to remove references etc from the header of the mail that goes as reply....

Also the root and the message must be in the same folder which slightly makes use of Threading a bit less popular at least from convenience point of view. In fact a better option of Threading would be to Have a "Related Messages" link which leads to a list of Messages related to the Message Being read...

Anup