Last week I was learned in deep the WCF channel architecture and how to development a custom transport channel. To put in practice these knowledge I had programmed a transport channel that works over POP3 and SMTP protocols.
This channel is based in the soap.smtp channel that Steve Maine write for WCF Beta 2, and is also based in the UDP transport channel that come in Windows SDK.
Here you can download the channel code and a complete sample of use.