When does the system put a received email in the spam folder? Does it have something to do with the subject line? I get a weekly announcement from one of my contacts and each time this particular item goes to spam. Other emails from her go to my in box.
What you are describing is known as a "false positive"; legitimate emails going to the Spam folder. When that happens, keep marking them as Not Spam to help train the filters. It could take quite a while to work. Something in those messages is tripping the filters. Keep in mind that other people could be marking them as Spam. There are a lot of variables that can flag something as junk.