I did it the way Thunderbird does it, and believe it to be a valid take (after all, could Thunderbird developers be *that* wrong?)
Do know that Gmail / web and Outlook do it the other way. Would like to provide an option, but have been busy with all the other stuff 
I still find it really funny that we have *smart* phones with GPS, super-powerful computers, distributed network time sync protocols with millisecond precision and yet.... Guess it's the same way with microwave clocks (is it so hard to put a backup battery there?)
I use Thunderbird on my main PC, and choose to display both dates (clearly only feasible if you have the screen real-estate for it). You can sort by either with one click on the column header. I find it is very common for emails to be in limbo for minutes, hours or days, and though I normally sort by the sent date, I occasionally switch to the received date. Sorting by the sent date gives a better idea of sequence of emails ( unless the sending computer has the wrong clock or timezone, as posted by someone earlier). I think sorting by either would be a valued improvement. My priority though would be for:
a conversation mode;
Add sender and all recipients of an incoming email to the people app or just the sender;
Sort by either date. Possibly show both on tablets. Chosen one in full, other may be partially eclipsed or hidden.
I appreciate that without a contacts function inside the app, you have less control over creation of groups etc. But I do also miss that. Absent the ability to create and manage groups, how about creating a NEW email to all (like reply all, but creating an empty Draft that will not sort into the conversation it came from). This would go some way to patching the absence of groups. In Thunderbird, if you reply all, then change the subject, it still sorts with the original convo, which is correct if the topic is drifting, but my suggestion is for creating a new topic to the same set of people.
