I have missed important messages because of spam. I can't comment further because I'm afraid of what I might say. Spam makes my blood boil.
I had a similar problem until I contacted my ISP (att.net) and complained. They said my account could be set up to go through Brightmail first but I had to check the box in my set-up. I went to the att.net website, logged into my account and checked the box. My spam went down 75% immediately.
You might want to check with your ISP if they have the same filter arrangemnet with Brightmail.