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.
SendGrid announced its latest endpoint for its Web API: v3/mail/send. The endpoint rounds out the v3 release that began last year. With the new send endpoint, SendGrid has realized its goal of complete API management via API. All of the v2 functionality remains available with additional features.