[osterhase]

I do not mean to rise this from the dead, but for the sake of completeness:

if(!is_dir($data['new']['dir'].'/sieve')) {
	mkdir($maildomain_path.'/sieve', 0700);
	exec("chown ".escapeshellcmd($mail_config['mailuser_name']).":".escapeshellcmd($mail_config['mailuser_group'])." ".escapeshellcmd($maildomain_path)."/sieve");
}

Thanks for your help!