The guys over at XYZ are taking a look into what we can expect from the future of Thunderbird. The 1.5 version is slated to be released in January 2006, & hopefully Calendar will be implemented sometime next year as well. I know that as soon as Calendar is out, I'm dumping Outlook for good..

"The program has often been derided because it lacks the scheduling feature of Lotus Notes and Microsoft's Outlook (Mozilla's lackluster Sunbird handles this) but its cross-platform performance is admirable and it has some very good extensions."