then you need to install spamassassin on the server and configure it, additional adjust postfix with pregreet and rbl checks. however, you then won't have the comfort of PMG. otherwise you need to change the mx (usually it's fast, for me it took just a few minutes to get alive), to prevent spammers coming from other "ways" I enabled submission (587) on my real server for sending users (could also be done through PMG, but I prefer not to do for some reasons) and changed port 25 and 465 to only accept mail from PMG. Works well. However, I use Plesk therefor.
