Can my boss not accept my doctors note.?

I had to call off work due to an problem that sent me to the doctor. I called in and let them know the problem and that i was on my way to the doctor. Today when I returned the said they don't accept doctors notes. Ive missed work in the past due to my son having a seizure and me having bronchitis. I have doctors notes for then to. But my employer said they don't take them. Im afraid their not taking them so if they decide to fire me they can say I never had a doctors exuse. they are doing this to all the employees. Is this legal?