Microsoft Entourage 2003

Microsoft Outlook 2002

Microsoft Outlook Express

An email client and personal information manager developed by Microsoft for Mac OS. It is fairly similar to Outlook.
 

A personal information manager from Microsoft, and is part of the Microsoft Office or available as a stand-alone program. PC & Mac.
 

An email client and newsreader included with Internet Explorer, the Windows operating systems and Office 98 for the Macintosh.

Mail.app (a.k.a. Mail)

Mozilla Thunderbird 1.0.2

An email client from Apple included in Mac OS X. Mail integrates with other Apple applications such as Address Book and iCal.

A free, open-source and cross-platform mail client for Windows, Linux and Macintosh.