Thunderbird has everything that you would want in an e-mail client and it is free to top it all. It has great POP3 support which runs across free e-mail solutions which don't even allow POP3 achess without an additional fee. Thunderbird offers extensions (plugins) that add loads of added functionality. Thunderbird is thoroughly maintained with up-to-date security threat busters added with Phishing and Junk filters, smart tagging for sorting e-mails and Firefox-like quick inline search.

There is one thing that Thunderbird does lack-a good calendaring feature such as the reverent Microsoft Outlook. Those used to Outlook's meeting invites and business cards will be sorely put off as Thunderbird doesn't have a complete solution for this feature yet. This feature is being taken care of in the 'Lightning Project' which is expected to add this functionality and much more. Nevertheless, those who don't care about the calendaring drawback can jump straight ahead and adopt the bird!

FOR: Easy migration, added functionality with extensions, several skins.

AGAINST: Cannot migrate back to Outlook, No Outlook-like robust Calendar.