If you literally mean you want an app to receive email then that means you're building an smtp server.
Luckily, someone already parcelled up an implementation of one of these things for you:
https://github.com/cosullivan/SmtpServer
Has to be said, there's a clue in the name there though.
Server.
This isn't really something you'd usually put in a wpf app and deliver to someone's desktop.
Alternatively.
You could "just" write a front end for a gmail account.
Google csharp email client and csharp smtp.
eg
http://csharp.net-informations.com/communications/csharp-smtp-mail.htm