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.
Context.IO, an e-mail API provider, has announced a "lite" version of its popular Context.IO API. The Context.IO Lite API is not a replacement for the original API. Rather, Context.IO Lite started as a small project to improve Webhook performance. The potential of Lite quickly made itself known, so Context.IO decided to fully develop the Lite version and has now launched a beta version.