KMail has lots of inbuilt features and has quite a few plugins as well. I like the mail composer window and its auto-correct features. KMail can import mail, filters and bookmarks from Evolution and Thunderbird. It has calendar integration with KOrganiser. However, KMail has drawbacks. First drawback, it’s deeply integrated into KDE desktop, which means, if you try to install KMail in GNOME, Cinnamon or Unity, you will have to install the full KDE desktop. I tried installing only KMail, but it kept throwing errors while setting up IMAP email accounts and composing emails. KMail does not support MS Exchange accounts as well.
Apart from that, KMail is buggy, does not import mail properly, its mail filters do not work most of the time (if there are too many emails coming in) and does not support signatures properly. If these bugs are quashed, KMail can give Thunderbird a run for its money. KMail comes bundled with the KDE desktop.