If you started to receive a lot of SPAM, or maybe the opposite and you don't receive letters you were supposed to receive - you should check SPAM filter settings.
The main option of SPAM filter is its sensivity, which is regulated by chosen number. The lower the number is, the strictlier filter is and higher the number is - the more letters you will receive! Which means, 1 - almost all letter will be received as SPAM, 9 - all letters will be received as normal.
Also there is a "white list" in which you can all your trusted e-mail addresses, and they will never be blocked, and "black list" - where you can add all e-mail addresses letters from which you would like to block!