Hi - I don't want to start a flame war, but I'm genuinely struggling (and have done for years) to find a decent email client for Windows.
In my opinion, and I believe this echoes Zimbra's efforts, search is critical. I personally also make use of tags. Beyond that, I don't have any particular email requirements, yet there aren't any great clients out there for Windows.
Outlook Express: Actually does IMAP very well, but tagging isn't present, and search is poor
Outlook (IMAP): This is frankly rubbish
Outlook (Zimbra connector): Better than IMAP, but I don't think the search exploits Zimbra's great server-side search functionality
Thunderbird 2: This is what I use, but it's a bit clunky in some places, and search is both slow, and requires you to choose exactly which fields to search.
Thunderbird 3: Rumoured to have better search, but it's not present in alpha2
Zimbra desktop: I don't think it supports winmail.dat files, and you have to page through scrollable lists, which seems to be a strange UI. It also uses a lot of disk space.
In contrast, on the Mac, OS X Mail is pretty good, and Spotlight is brilliant.
So - what I'm looking for is basically Thunderbird that's got the search capabilities of Spotlight. Does anyone know of anything, or is anyone using another mail client with great success?