Admin --> site settings --> user account settings --> use email address as username

That should work.

Mark

Thank you, Mark. The reason why I could not find this option anymore is that I chose "Custom" as "Registration Form Type" in the meantime - which makes this option disappear. Not sure why, but I think if "Username" is left out in the custom fields, then this will have almost the same effect.

Thank you for the warning, Michael. This made me change my mind and I decided that it is better to register the user with username, password + email address. Works fine when I select only those fields in custom mode.

Drawback: when I do not have the user input a displayname, then the user.ascx skinobject will not show anything in logged in state. So now I wonder how to correctly derive from the code in user.ascx.cs. Only a minor code change is required but I want to only make this change for one skin and not touch the original skinobject.