Surely you're joking. First, Outlook isn't for anyone who'd be comfortable operating mutt. Period. Second, sure, Outlook has stupid design-decisions. But nobody claimed that Outlook is perfect, it was argued that Outlook was better than Lotus Notes which is was competing against when it was launched. And it was (and is), leaps and bounds. Third, Outlook isn't just a mail client, it's an integrated communications suite. It has taken OSS years and years to catch up with this. You may not agree that it's a critical feature, but for millions of business users, it's a deal-breaker.
There's good innovation in the kernel-space, too, but user-facing apps are playing catch-up.
KDE has had very good PIM software for a while now even before KDE4. Kontact has always been a suite for that kind of stuff, and it's possible to do most things in Outlook in Kontact. Also if you're talking about Exchange and groupware servers there are a few for Linux like Zimbra. However I haven't even seen Fortune 500 companies make most of the use of Exchange features.
Also your measure for success is very shallow. Essentially you don't see OSS as competition, which belies your actual position and tells me that there's no point in arguing this with you considering your mind is made up about OSS. As far as I'm concerned all vendor software loses out because it's missing basic features from a IT management standpoint. Also from my experiences millions of business users are too stupid to use Outlook in any shape or form because they lack basic computer proficiency skills.