Thunderbird is a email client that belongs to the Mozilla family of open-source software. It is my favourite because of the ease of use and fast performance. Thunderbird does not support the Microsoft Exchange Server protocol. In order to access the email at my workplace, which adopts the Microsoft Exchange Server, I have to set it to use the imap protocol for receiving emails, as well as the SMTP protocol for sending out emails. One great thing about imap is that it also supports email folders. So you can create folders and move emails into the them just like what you can do in Microsoft Outlook.
The downside to Thunderbird is that it does not support the calendar and appointment functions like Microsoft Outlook, though these are not big issues for me.
As for alternative to the global address list available in Microsoft Outlook, Thunderbird supports LDAP, which allows you to configure and access address list based on LDAP.
Other good points about Thunderbird include its support for gmail. With the Webmail add-on, it can also be configured to support yahoo mail and hotmail, which normally has to be accessed via the web browser.
Evolution is another popular email client in the Linux desktops. In fact it is the default email client in most of the Linux distros. One big plus point about Evolution is its support for the Microsoft Exchange Server. However, I have tried for quite sometime before getting it to work with Microsoft Exchange. In my case, the trick is to select "Exchange MAPI" as the server type when setting up the Microsoft Exchange email account. Previously I have tried selecting "Microsoft Exchange Server" and it simply refused to work.
Other than the sending and receiving of email, so far I have only tried out the global address list, which was working well. I have not tried out the calender and appointment functions though.
For the sake of completeness I shall just briefly mentioned KMail though I have not really tested it out before. KMail is currently the email module of Kontact, which is the suite of KDE-based applications that deals with Personal Information Manager (PIM). Other modules of the suite includes KOrganizerAkregator - Read your favorite feeds which handles calendar and scheduling, KAddressBook which manages your contacts, and Akregator which is a feeds reader.