E-mail confirmation for registrations (Double opt-in) can be enabled for the website using the Registration requires e-mail confirmation setting, which can be found in Site Manager -> Settings -> Security & Membership. For additional information about this functionality, please see Registration approval and double opt-in in the Kentico CMS Developer's Guide.
Help URL: http://devnet.kentico.com/docs/webparts/index.html?RegistrationConfirmation_overview.htm