There's one area of UI that really needs work.
Hands up all those with users (of any client) storing their read mail in Trash/Deleted.
Hands up those with users - or themselves - storing read mail in the Inbox thumbs up icon is the nearest for myself).
Inbox should be for incoming, unread mail only. Trash is for stuff that's going to be deleted (end of day, over a certain age, over a certain byte limit - whatever). If you want to keep something about while you mull over it a Pending tray would be appropriate. For everything else we need a filing system that's a good deal better than the existing archive/filter system.
For instance instead of just using threading for display of linked emails in the same folder, make a folder for them.
- Automatically link sent and received emails that belong together instead of leaving sent in their own folder where they don't even get threaded with received emails for display.
- Automatically put mails to/from particular individuals, domains or addresses grouped in the address book into appropriate folders.
- Do that when the user moves onto a new mail in the Inbox if they didn't delete or pend the old one and do it with sent emails.
The papered office has known about manual filing for decades if not centuries; the paperless office (sic) should be able to automate it. Done well it might even be able to organise me.
The next step might be to extend that filing system for emails into a fully-fledged document management system covering emails and other documents.* If not done within the email client provide hooks for an external manager.
Yes, it might come as a shock so, as with any UI change, make it optional, at least for a long time, and don't default to it in the first releases.
* Remember that at least some of the documents relating to a topic will arrive as attachments, others won't but they need to be managed together.