Random Riddle

There is no mail on a Sunday!
Hold your mouse over for the answer.
On a late Sunday afternoon in a mansion, there was a dead man found in the library. The people in the house called the police, the police called a professor, the professor called the people who were working in the house. The cook said he was cooking, the maid said she was cleaning up, and the butler said he was collecting the mail. The professor told the police to arrest the butler.

Why didn’t he arrest the other three?