Re: Picking the data to delete
Seamonkey also makes provision for this and I use it like that. It's a faff to set up so the average punter will not do it. That's why so many system admins report users storing previously read mail in either the inbox or in the bin. It's been commonly observed user behaviour for years.
It's also possible to ensure that Sent mail and incoming mail end up in the same folder so that they get properly threaded. It just doesn't happen out of the box. Of course suggest that it does and some Stockholm Syndrome sufferer will be along to tell you that that's not how they work because they've been used to having something so useless that they regard it as normal.
Is it not beyond email client developers to produce something a bit more joe-public-user friendly?