Thunderbird doesn't yet support all the workflow the Exchange/Outlook combination, for example calendar events. On the Linux Desktop Evolution is a better pick for full workflow integration, but it's not portable across Windows/Linux like Thunderbird is.
For client e-mail alone, Thunderbird is definitely an option. It supports IMAP, POP and LDAP and a host of integrated Windows standards, while you still need windows. And you can use the same IMAP server from Windows and Linux if you want. That means the same account.
Some people worry about the safety of their e-mail when using a product that's not yet version 1.0. Thunderbird's 0.7 version is an honest statement of status, whereas a commercial version 1.0 is less honest. In my experience, the email storage systems that Thunderbird uses have been very solid and reliable for a long time.
This was first published in August 2004