How to limit user registration to a specific set of domains?

Question

I'd like to limit registration to some of my sites based on the email domain of the user. For example, I only want people with @gmail.com emails to register.

Any ideas? Plugin possible?

Answer 1

Tough task...

function wpse27756_restrict_register_email( $user_email ) 
{
    $errors = new WP_Error();

    if ( ! preg_match( "/gmail/i", $user_email ) 
    {
        $errors->add( 'invalid_email', __( '<strong>ERROR</strong>: You are not allowed to use other mail accounts than GMail.' ) );
    }
    return $errors;
}
function wpse27756_add_register_email_filter( $user_email ) 
{
    add_filter( 'user_registration_email', 'wpse27756_restrict_register_email' );
}
add_action( 'init', 'wpse27756_add_register_email_filter' );

Just written straight out of my head without testing. Give it a try and let us know...