Having built several in multiple languages, I'd recommend for anyone to build their own email client.

It's quite easy to get the basics up and running but quickly becomes tricky getting all of the messages to display consistently. Then you can add embedded images and attachments to the mix. The list goes on and on. But, you really will learn a lot about protocols, parsing text & binary data, working with legacy formats and just all kinds of interesting stuff. Even if you don't wind up with an amazing client, it's still a lot of fun!