The MyNewsletterBuilder Perl SDK by MyNewsletterBuilder allows API interaction to access email marketing capabilities. With the kit, that is available for download, developers could create online campaigns with newsletter services integrated.
Guerilla Mail is a simple idea with one purpose: to avoid spam. The idea is dead simple: go to the website, it gives you a random e-mail address that's good for one hour, and links to nothing else. Register for sites or whatever with it, and then never worry about it again. Although the web service is convenient, sometimes it's not everything you need. For those cases, one might want to look into using the Guerilla Mail API.
DokDok, a Montreal-based startup, has had an email attachment management application on Google Apps some time. To make this work, it made an API to use internally that intelligently uses email accounts as if they were datastores. Recently, the company announced that this API would be renamed Context.IO and available to all.
Email, which is one of the oldest services available on the internet, is a key mechanism via which you can reach out to your users. Over the last year, the number of services providing this infrastructure as a service has grown to include 6 email sending APIs. The newest, Amazon Simple Email Service, validates the market.