It's a shame about KMail, because it used to be a good e-mail client. I have KDE on my main laptop and am quite a fan of KDE as a DE, but some of the official KDE apps never recovered from the KDE 3 to 4 switch.
I would be lost without Thunderbird. I use it extensively in several time zones for work and personal e-mail accounts, and have thousands of e-mails stored in it for many years. It manages six e-mail accounts, two of which use the excellent FOSS DavMail to access Outlook Web Access WebMail accounts of two companies, and I use the Lightning integrated calendar which correctly stores meetings and events (even from the OWA accounts). And, as I dual boot with Windows 7, I have the Thunderbird e-mail database located on the Windows NTFS partition so that I can access the stored e-mails and send e-mails from both Linux and Windows.
These are the Thunderbird Extensions that I find invaluable:British English DictionaryChange quote and reply format
<<< Change the reply header and the reply format.ConfigDate
<<< Configure the 'Date' column and displayed date in message header.Correct Identity
<<< Allows you to configure the identity that will be chosen upon composing mail.Diccionario de Español/EspañaEnigmail
<<< OpenPGP message encryption and authentication.ImportExportTools
<<< Adds some tools to import and export messages and folders.Lightning
<<< An integrated calendar for Thunderbird.Provider for Google Calendar
<<< Allows bidirectional access to Google Calendar.Test Pilot for Thunderbird
<<< Help make Thunderbird better by running user studies.Timezone Definitions for Mozilla Calendar
<<< Timezone definitions required by Sunbird and Lightning.Verificador Orthogáfico para Português do BrasilWebMail
These are the Thunderbird Plugins that are installed:DivX Browser Plug-inDjViewJava Plug-inmplayerplug-in is now gecko-mediaplayerQuickTime Plug-inRealPlayer 9Shockwave FlashWindows Media Player Plug-in
A couple of old bookmarks: 12 Thunderbird Addons You Shouldn’t Be Without
, 10 must-have Thunderbird Addons (+ 25 more)
There are still several other handy Thunderbird add-ons I want to try when I get around to it.