Your LORs should all be on letterhead. Some schools are rigid about this and it will hold up your app. When I was applying I had to contact a school to see why I wasn't complete. At that point, they informed me one of my LORs was not on letterhead and would need resent on letterhead. This was a letter from a PI whom I worked for. Not sure what happened, I am sure he knew this should be on letterhead... maybe he emailed to an assistant and told them to print it out and mail it? Anyway, an LOR from a professional (and you wouldn't ask for one from a non professional) just should be on letterhead...