Anti SPAM configuration for plesk 8.6

I host a plesk server 8.2 with godaddy. I have around 30 domains with 250 mailboxes total. Users complain from SPAM and when I adjust settings, they complain of losing emails. I need to minimize SPAM using SPAM Assassin or another plugin. I will need 2 options, with pros and cons, price (monthly monitoring service is cost prohibitive) and also, configuration and installation if applicable. I might consider a monthly monitoring service for my server if the results are satisfactory. I help a small business based in Mexico, so, budget is limited.