I'm not an expert in Outlook, but reading the article I can see the sort of thing that's being referred to. It's not email per se, but the groupware side that's lacking. One example - the ability to schedule a meeting at a time when all particpants are free and then put it in their calendars - all without the participants having to do anything.
Centralised address lists, public folders etc are other examples.
Central address lists are easy to solve with an LDAP server. Anyone with the correct rights to the "Contacts" tree (ou) of the LDAP database can add and edit contacts. The LDAP server can also manage all the mail logins. If you run a decent IMAP server (Cyrus for example) you can also manage public mail folders. The only thing that is lacking on the Mail Server side is an "integrated solution". There are some promising web administration projects (coming out of US Uni's with deployments with more than 20,000 users) for Cyrus though. This doesn't solve the calendar server problem though. A project like Hula might be a good solution for that. Companies want a point and click solution. There is a good opportunity for an Open Source solution based "mail application server appliance". Some enterprising people could make some good money by providing a box that companies just "plug in" and get a good mail/calendaring/contacts system. Need more capacity? Just add another appliance. This is what Barracuda Networks are doing very effectively in the spam filtering space. Regards
In other words, it's all the centralised stuff which is managed by Exchange. Corporations, of course, like centrally managed systems.