http://www.getthunderbird.com/

Thunderbird is a free, extensible mail client software with excellent IMAP support.

See also POP vs. IMAP or Why I Use IMAP

http://luxsci.com/blog/head-to-head-battle-of-the-email-clients.html

https://support.mayfirst.org/wiki/faq/email/speedup-thunderbird-imap

http://luxsci.com/blog/optimizing-mozilla-thunderbird.html

http://bleedingedge.com.au/blog/archives/2005/11/making_thunderbird_fly.html

See also Thunderbird Synchronization

Manage identities:

Tools → Account Settings → Select an Account → Manage Identities (button)

You can set a different signature block (and other settings) for each identity you configure.

Once you create multiple identities, you can select which identity to send messages from by clicking on the From: address in the compose window.

There are various add-ons available for Thunderbird to extend or change the default behavior:

https://addons.mozilla.org/en-us/thunderbird/search/?q=identity

http://enhancedlinux.com/2010/01/15/disable-thunderbird-3-bloat/

Thunderbird caches local copies of your message store in your Windows profile. If you have Roaming Profiles, this can negatively affect your login/logout times.

However, the local caching is a significant speed enhancer.

Instructions adapted from here:

http://kb.mozillazine.org/Moving_your_profile_folder_-_Thunderbird

These instructions will seem strange since there is no “move profile” or “register profile” button in the profile manager but it isn't a mistake. When you press the “Create Profile” button and then browse to your profiles location it does not do anything to your profile, it merely updates profiles.ini to point to that profile so that Thunderbird can find and use that profile. You could do the same thing by editing profiles.ini yourself but it is very easy to make a mistake doing that.

Use the Profile Manager to move your profile:

1. Close Thunderbird.
2. Copy the profile folder (in XP: C:\Documents and Settings\username\Application Data\Thunderbird\Profiles\profile name.default) to where you want to store it. (You can copy it anywhere, except the program directory.) I chose c:\Mail\profilename.default
3. Start the Profile Manager. Usually Thunderbird isn't on the path, so you will probably need to specify the program location when you do that. One way to do that is to open the “Run” field. Copy the contents of the Target: edit field in the shortcut used to run Thunderbird and then add -profilemanager to the end in the “Run” field.
4. Click on the “Create Profile…” button.
5. In the dialog that opens, enter a descriptive name for the profile.
6. Click on the “Choose Folder…” button. A “Browse for Folder” dialog will open. Select the profile folder you copied and click OK.
7. Check that the path shown in the “Completing the Create Profile Wizard” dialog is correct, and then click “Finish”.
8. Select the new profile and press the Start button to have Thunderbird use the moved profile.
9. If most of the folders for an account aren't visible and your Inbox looks empty, find the local directory setting at the bottom of the Tools → Account Settings → Server Settings page, and use the “Browse” button to update its location. Normally it points to a directory named after the mail server used by that account, unless you using a global Inbox. In that case it points to the “Local Folders” directory in the profile.
10. Once you have tested the moved profile, delete the original profile from AppData to clean up your roaming profile.

Edit (Tools) → Account Settings → Synchronization and Storage

Delete cached files taking up space:

C:\Documents and Settings\<profile>\Application Data\Thunderbird\Profiles\<profile>\ImapMail\*