1. Spam Filtering - The junk mail control, a Bayesian filtering implementation - sucks. Messages that I mark as not-junk continually get put in the junk folder. It just doesn’t work. With the rise of crowd powered technologies
  2. Calendar Integration - Is a start, but it’s still pretty poor. The UI for it is ugly, when you add an Outlook appointment, it won’t remember that you added it each time you open the invite, etc…
  3. Slow Search - GMail rules because it focuses on being able to quickly search through your email. This is hard to do with a desktop computer, but by learning the behavior of users, it can optimize its algorithm tremendously.

Besides having more resources focused on Firefox, the problem with Thunderbird is that instead of “writing the best e-mail client we can” the focus is instead “write an open-source Outlook.” What made GMail the most popular webmail (and e-mail in general) provider was not because they offered 2 GB of storage up front… it was because they didn’t try and make an Outlook clone (which at that time every webmail interface was). If Thunderbird would do that (as Firefox did with the browser), I think we’d have an amazing desktop product.

But personally… I’m just waiting to buy my own server and install Zimbra.