Thunderbird should compete with Outlook. I was so excited for Thunderbird 3.0 for it to have the possibility of the Calendar being integrated into it, but no. I was let down and still don't really see how Thunderbird 3.0 really any better than 2.0. No offense, I hope Thunderbird will get on track and start competing for Outlook users. Enterprise users.
Thunderbird should include the following or have Add-ons (Calendar should NOT be an add-on, it should be integrated)
Calendar - set alarms, appointments, etc, cause your email is almost always open.
WebCal/CalDAV support - needs Calendar
Tasks/ToDo - its in the calendar project already but migrate it into Thunderbird.
ECS/UCS - Enterprise/Unified Contact Service; be able to connect it up to a server, like AD, that knows your contacts or has it listed like in a domain type setting... like the GAL does in Outlook (global address list). But be able to section it off saying - xyz email account can add this and find contacts in Thunderbird.
Presence Indicators - if messaging is integrated, like Communicator 2007 R2 does for Office.
The email clients I use cause of their features, usability, and expandability are:
Zimbra Desktop (free) cause it's featured like Outlook - even allows for plugins for like Meebo, etc.;
eM Client - again Outlook like with some Messenger integration;
Kontact (KDE Office Suite) integrates into everything - has journal, Atom feed readers, tasks, calendar, email, etc.
Evolution - again Outlook like - available for Windows (buggy) and standard on Gnome versions of Linux - Ubuntu.
Windows Mail - it includes a calendar and is starting to become more like Outlook (free).
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