You've Got Spam!
Obviously, e-mail and groupware (which tends to include calender, e-mail, address book, and to-do functions) are options that many businesses use. The Windows-world de facto application for this is Outlook.
For now, consider that KDE includes the very good kmail application.
On the Gnome side, there is Ximian Evolution
products/evolution/), which not only replaces most of Outlook, it also plays nice not only with standard RFC-821 compliant mail servers, but also with the proprietary Lotus Notes servers and Microsoft's Exchange Server.