I use Thunderbird as my desktop Email Client because its very light, fast, open source and supports add-on. With the launch of Thunderbird 3.0 more features have been added and is now simpler to set up an email client.
If you just add your email id, Thunderbird will automatically detect SMTP / POP servers and set up email client in few seconds.
And there are add-ons which will display images when important mails are received. You can know more about it in my older post.
Now, you must have observed that whenever we minimize Thunderbird it settles down in taskbar. Unlike any other email client which can be hidden in System Tray. This is will create a problem when you open up many program windows, leaving no space in the taskbar.
I tried to find a solution to this problem and finally ended up using an addon named MinimizeToTray Plus.
This simple addon once installed will minimize and hide Thunderbird to System Tray. You can also view and compose mail with a single right click on Tray Icon.
MinimizeToTray Plus addon is also compatible with other Mozilla Products like Firefox and Sunbird.