Make sure sieve is running on port 2000 and you can use this config option to
rewrite hosts if you use a proxy:

$config['sieve_rewrite_hosts']='localhost=imap.example.com,imapproxy=imap2.example.com';
$config['sieve_vacation_subject']='Optional globalvacation subject';
$config['sieve_usetls']=false;
$config['sieve_port']=4190;