Now Lotus Notes is dead. Exchange/Outlook at the current front runners, and they are producing really lazy email clients and not really pushing email forward at all.
What can be done? No clue. The only organisation seemingly able to compete with Microsoft in this arena is Google with their Google Apps for business/education offering. But that has its own issues.
At least Microsoft has made "Outlook Online" a lot better in recent years thanks to their push for Office 365 Subscriptions.
PS - Search in Outlook is shockingly terrible. For some reason during the Vista era someone at Microsoft said: "Let's make search terrible in all of our products." So Vista's search was junk (read: replace a nice Windows 95 search sidebar with hidden obscure magic keywords which didn't work, and only index "known" file types (which is none)) and that filtered down to Outlook which got a huge bloated indexer which is inconsistent, slow, and still relies on magic keywords to accomplish anything. PPS - Thunderbird also has a terrible search indexer, however it also offers simple filtering which is wonderful. You can disable what they call their "global indexer" (which you should, it sucks) and then just use the "quick search" box within each folder, it is insanely quick, and the filtering options offered a huge efficiency gain.
Win 7 & 8, Outlook 2007 & 2010, respectively. FWIW.
The PST file fuckery is the one thing I wish it handled differntly. After that it would be the mess of things caused by 'borrowing' from Word/IE when it comes to composing and viewing emails.
It seems to me, that nobody really can write a real good eMail clients.
Of course they could fix MS Word's HTML output and fix Outlook to use MSIE's rendering engine ... but that'd only reduce lockin by allowing Outlook users emails to be read properly in other MUA. Never going to happen.
I believe up to 2003 this was an option that was not enabled by default too.
e.g. an known Outlook 2010 bug (with Exchange 2013) crash the Outlook client on startup. But you can still open *.msg files, it opens the mail in a window and one can edit the mail.
It's all about Bill Gates vision "information at your fingertips" from the early nineties and the Cairo operating system.
Watch this video from Bill Gates Comdex '90: http://youtu.be/kL8zoQVJaD8?t=19m26s
Cairo '91-'96: https://en.wikipedia.org/wiki/Cairo_operating_system