As much as I love the Plasma Desktop and other KDE software, I found Kmail and the entire PIM (personal information management) to be in utter contrast with the rest of the KDE family.
Kmail is extremely rich in features, but it is also extremely complex to configure and use. It's also very hard under KDE to get all the components - calendar, chat client, address book and email client to work together just by configuring one account.
If you don't bother with the rest of the components and just want to use an email client with just one email account, you may give Kmail a try. If you have more than one email account, configuring them would be a hair-pulling experience under KDE.
If Kmail is so bad, why am I listing it among the best email clients? The tools themselves (Kmail, Addressbook, etc.) are excellent -- even better than the other email clients I've mentioned. But for some reason the developers decided to make it extremely hard to configure.
Kmail seems to have been developed without the user in mind.
Whenever I manage to get Kmail to work, I love it and I only hope that since developers are working on Plasma 5, they will focus some resources on Kmail and its better integration with the rest of the system.
Pros: Extremely feature rich and customizable
Cons: Good luck configuring it
Download and install Kmail