Plesk CentOS Server troubleshoot/maintenance

hi, we have a dedicated server running Plesk and seem to be having a few issues that need resolving. We recently have been finding that Gmail (and others) have been flagging emails from our server as spam (it says that it has detected large amounts of spam coming from our server). To combat this we installed the plesk outbound spam protection module. At this point our Email stopped working (inbound and out bound). I notice that when trying to restart the postfix service we get an error. I am not sure if this is the only issue but there is a problem somewhere. We require a server admin to check all our dns/mail settings to ensure that our server is secure and working correctly. We also have a permissions issue with regards to FTP and the files that live on the server. We are unable to modify or delete certain files due to the group that they belong to. This issue also needs fixing. Please feel free to ask any questions, many thanks, anthony