The Customer.io C# SDK by Digital Creations communicates with the API to integrate customer interaction data. Resources include installation and usage. Developers can implement the ICustomerFactory interface with code available on the site.
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.
Email is one of the oldest mediums of communication on the Internet, and, for many companies, it's still one of the most important. Delivering email and tracking email campaigns reliably is crucial to success for countless businesses today.