Who Is The Murderer?

Author: NEO JIA QI

Riddle: A man was found murdered in his own house on a Sunday morning. His wife called the police. The police questioned the wife and got the following alibis: The wife said she was sleeping. The Cook said he was making breakfast. The gardener said he was picking vegetables. The maid said she was getting the post. The Butler (a personal assistant) said that he was cleaning the closet. The police instantly arrested the murderer. Who did it and how did they know?
Answer: The maid. There is no post on Sunday.
Who is the murderer? Riddle Meme.