They are being territorial and such behavior can get dangerous to both cats. If your cat is fixed, you should not let him out because of this cat. Feral cats are usually unfixed which will cause your cat to start peeing in and around the house to mark his territory. Also, it can make the other cat more dangerous to your cat. An unfixed cat has more male hormones. Also, a fixed male, under most situations, loses his desire to fight, but the other will fight if he feels he can win. Your cat may also be at risk of other strays who will smell the situation for miles and will respond. Another thought is that you might want to keep your cat away from strays because they carry many diseases, such as feline leukemia, which can be passed through scratches or bites.