"So why no FOSS challenger to that ?"
Because it's an expensive piece of software to develop without any real return. Heck, in Linux you still get separate software for SMTP and POP/IMAP instead of integrated solution - do you expect they would also integrate all the other functionalities Exchange delivers? Moreover Exchange plays well with Active Directory, so you don't need to manage users and mailboxes separately - and Linux can't still agree on a single, powerful, default identity management system easy to setup and use.
Last but not least, with the distro and desktop fragmentation, delivering something alike Outlook is quite impossible. Again, it's a complex piece of software with a dedicated UI which requires a not small development effort.
And one of the few Linux backers until late was IBM - which had all the interest in selling notes....
It's the same old story, Linux fails to deliver advanced software because there's little interest in investing in it as long as you have to give it away for free.
Biting the hand that feeds IT
