Ultimate Member Registration - List "disallowed" domain extensions such as .ru and .in

um-registration-email.php

<?php

add_action( 'um_submit_form_errors_hook__registration', 'um_custom_validate_username', 99 );

function um_custom_validate_username( $args ) {
    static $domain = '.in, .ru';

    if ( isset( $args['user_email'] ) ) {
        if ( str_ends_with( $args['user_email'], '.in' ) || str_ends_with( $args['user_email'], '.ru' )) {
            $message = sprintf( __( 'Cant use email domain %1$s for registration', 'ultimate-member' ), $domain );
            UM()->form()->add_error( 'user_email', $message );
        }
    }
}