As far as I know, there are no equivalent objects for WinRT.
Do you really need those object's though?
MailAddress doesn't really add any value other than allowing you to set a Display Name along with the email address. The header for a Email with a Display Name for "From" looks like:
From: Joe Dirt <joe.dirt@somegmail.com>
You could implement your own MailAddress easily enough if you wanted to, but are you sending emails directly from the device?
ContentType, again these's only a handful of content types you really want to send/receive on a Windows Store App.
json/xml/html, and possibly some media for video/audio. But depending on what you're building, it's unnecessary considering the header value for JSON is application/json
or application/json; charset=utf-8
if you want include encoding.