You don't need Postfix for CiviCRM. A standard install of CiviMail requires Postfix and Amavis only apparently, so you won't be able to do this on a Plesk server. There are alternative (experimental) install instructions for CiviMail however that don't even require root access and work with any MTA: http://wiki.civicrm.org/confluence/...tive+configuration+-+no+Amavis+or+root+access