Postbox email client makes Thunderbird more social
You can easily import your contact list from Outlook or Thunderbird into Postbox. And when you login to Facebook, those contacts will be added as well. Postbox indexes your messages, contacts, images, links, and attachments, making it simple to search for items stored in your emails.
There's a to do list, but no calendar. And there's no support for plugins, which means you can't use the Lightning Thunderbird extension. But Postbox is still in beta, so perhaps we'll see a built in calendar or add-on support down the road.