UltimateMember whitelist domains - The terse version

add_action( 'um_before_new_user_register', 'require_whitelisted_email_for_signup' );

function require_whitelisted_email_for_signup( $args ) {
	foreach ( [ 'yahoo.com', 'gmail.com', 'hotmail.com' ] as $domain ) {
		if ( str_ends_with( $args['user_email'], '@' . $domain ) ) {
			return true;
		}
	}
	exit( wp_safe_redirect( add_query_arg( 'err', 'blocked_domain' ) ) );
}

function str_ends_with( $haystack, $needle ) {
	return ( strripos( $haystack, $needle, - 1 ) === ( strlen( $haystack ) - strlen( $needle ) ) );
}