The Pepipost PHP Sample Code by Pepipost demonstrates how to work with the SDK. Lines of code show API key, sender, recipient, subject, and HTML text. Requests and responses in JSON display how to generate sessions.
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.
The number of services offering real-time APIs is slowly but surely expanding and it looks like we're going to have to add quite a few more. Since the start of the year a new type of service has started to appear--client push services, which help developers include real-time updates in their web apps.