The Contactology C# SDK by Contactology allows to integrate email marketing features into web services and mobile applications. The only requirement according to the official site, is to work with JSON .NET. Sample code available to illustrate API interaction.
Additional third party integration with Gmail has been possible (you only have to look at all the contact import features from sites like Facebook), but these have either been clunky manual procedures that involve exporting contacts to a CVS file, or they require you to supply your email password. Neither option is all that attractive. Now there is an alternative. Google have recently added the ability to access the IMAP/SMTP features of particular Gmail account using OAuth.
ZeroBounce upgrades its global data center infrastructure by deploying high-availability clusters in the United States. The new data center responds to the growing demand for real-time email verification and is intended to better serve U.S. customers through the API.