Open Source company grade alternative to Outlook?

This is not for me personally but for everyone searching this in a search engine, so this is a community wiki question.

This question is based on this answer in which he stands that Mozilla Thunderbird is a alternative for Outlook. While Thunderbird is great for SoHo I simply disagree that it's an alternative to Outlook at company level.
In fact I would say that there is no good open source alternative for Microsoft Outlook (+Exchange) which can be used to entirely replace Outlook company wide.

I would like to hear your opinions on the following questions:

• Is there such an open source PIM suite (+server) which can compete with Outlook (I can even be web-based, as long as it's hosted internally, so no Google Mail/etc.)?
• If there is one: Have you personal experience with using and/or migration? What are key points?
• If not, why is this missing?

Zimbra seems to be what you are looking for

Not open source, but you could consider Google Apps for Domain. Not directly comparable to Exchange/Outlook, but if you consider this alternative and the company is willing to change the way the company works, this will give additional benefits from a collaboration viewpoint.

Zimbra - http://www.zimbra.com/ . The most promising of them all. Now owned by Yahoo.

Open-XChange - http://www.open-xchange.com/oxpedia

Open Groupware - http://www.opengroupware.org

Also there used to be hula-project which Novell infamously dumped in 2006. AFAIK, the project seems to have dead and there is no active development happening now. The source code for that is still lying available via SVN from Novell's servers (http://developer.novell.com/wiki/index.php/Hula_Subversion).

And also there is the Kolab Groupware project. (http://www.kolab.org). You can use it along with a Horde Webmail and Calendar. It can also work with a variety of other clients including Outlook, Kontact, Thunderbird.