its also a very good idea to have  option to switch between folder view and threaded view ,,,why?

1-becouse some of the messages aren't actually related together but they are from the same sender, 

and some of the messages may content an bank pin number for example and u may want to access this message directly so u want to save it in customized folder created by your self

2- Draft folder very important folder and must have. for example: you are writing message and have to get a number or link or whatever from another app or even from outside,  so u save the massage till u get what u need or till the time is correct to send it maybe after 2 day u should have the ability to save another one without erasing that one this is what called draft folder.

So it may be as follows

U have the inbox folder , out-box , sent-item , and draft with ability to create a new folder to sort some of the messages by your own but in the inbox folder u got the option of having the messages sort by date or by sender witch it (threaded view)