Mailbird 2.0 supports separate apps that can extend the programs functionality such as Sunrise Calendar, which allows one to add multiple calendars in one view or Evernote which allows for easy not taking and sharing.
This e-mail client is free and very simplistic as it tries to only display the most necessary things (email list, labels, contacts, add attachments, and etc.). As it is lightweight it is quite fast and definitely shouldn't really slow down your computer..
The Inbox Analyzer, which organizes your newsletters and contacts using folders and filters works quite well. There might be some mislabeling from time to time - a friend gets labeled as newsletter etc.