Looks aside, Kontact has all the features you'd expect of a modern email client. Incoming messages are flagged in the KDE Notifications panel, it has unified mailboxes, and, like Thunderbird, it will spot words like 'attached' in your message body and, if you've not attached a file, check that that's not something you've overlooked. That's not the only similarity to Thunderbird: open the Find Messages dialog and it's all but identical to Thunderbird's Search Messages pane, through which you can stack multiple criteria to zero in on a handful of matching emails.