I turned their spam filter off. I'd rather have spam than have lost mail.
While I used it, over 100 mails got lost in the space of two months. I do say "lost" because they delete the blocked mail - unlike Google which puts it in a "spam" folder for you to browse "just in case". Oh, did I mention they had activated this filter without notifying me? I had to manually turn it off! Man, I was pissed off. This was during my unemployment time, and I had received several emails from companies that got nuked by my ISP.