Battle of the Ajax Mail Packages
Traditionally, there have been two paths to choose from when considering mail servers. The Redmond path was some variety of Microsoft Exchange Server with Outlook as the client, and possibly POP3/IMAP and Web mail as a backup when out of the office.
The other path, the path of the penguin, was Sendmail or Postfix, or possibly a more obscure mail transport agent (MTA) with POP3 and IMAP as the connection to the mail client of your choice. If you wanted Web mail, you'd use a package such as SquirrelMail running under Apache. There were, of course, other choices, such as Lotus Notes, but by and large, most e-mail installations used one of these two solutions.
I'm sure there are many die-hard Linux folks out there who are silently saying, "who cares?" But the reality is that in most corporate-IT environments, Outlook and Exchange are a well-entrenched aspect of the company mentality. And, it's hard to blame companies for clinging to them. The terrible twosome are full of useful features, such as meeting and calendar integration, that make them highly useful. On the other hand, it would be difficult to find a Windows sysadmin willing to describe administering an Exchange server as a pleasurable experience.