Spam, spam chips and spam

[Leftie]

This last few days I have missed so many important emails because my Spam filter has gone mad. I set up on my ISP force9 to ID spam and mark it accordingly, then set up Outlook to delete anything with 'spam' in the title. Worked great, I was getting about 300 emails a day, the filter was taking care of 80% and I was able to sort the rest. Then earlier this week everything was being marked as SPAM and I have now had to turn off the Outlook rule and manually sort all the emails. I went to Outlook and set the junkmail filter to low, and then to Force9 but they only have a spam filter on/off choice.

is it something I have done do you think, and how can I undo it? I did send a large 'Christmas' message to most people in my address book, could that have triggered it?