Microsoft's Office 365 APIs can be integrated into 3rd party applications. In order to do so, the app must use Azure AD authorization to confirm accounts. This page describes how to add Microsoft Azure Active Directory common consent to an application written in ASP.NET app.
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.