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add_action('um_custom_field_validation_user_email_details','um_custom_validate_user_email_details', 999, 3);

function um_custom_validate_user_email_details( $key, $array, $args ) {

    if ( $key == 'user_email' && isset( $args['user_email'] )) {

        if ( $args['user_email'] == '' ) {
            UM()->form()->add_error( 'user_email', __( 'E-mail Address is required', 'ultimate-member' ) );
        } elseif ( !is_email( $args['user_email'] ) ) {
            UM()->form()->add_error( 'user_email', __( 'The email you entered is invalid', 'ultimate-member' ) );
        } elseif ( email_exists( $args['user_email'] ) ) {
            UM()->form()->add_error( 'user_email', __( 'The email you entered is already registered', 'ultimate-member' ) );
        }
    }
}