E-mail based user authentication is described herein. A user can access resources of a service provider by submitting only an e-mail address to which the user has access. The service provider generates an authentication ticket corresponding to the user's login request, and transmits the authentication ticket to the e-mail service provider indicated by the submitted e-mail address. The e-mail service provider processes the authentication ticket, and enables either approval or denial of the authentication ticket, whether by explicit user action or by automated processing.