Ford showed a copy of the typo-ridden note to reporters this afternoon, saying "this is serious."
The mayor says police have been informed about the email, which was received by his brother, Coun. Doug Ford.
Police say they received a call about the email Sunday night and add that officers are currently at City Hall.
The threat warns that city hall will "disappear from the map" if the mayor does not quit.
The controversial mayor has faced numerous calls for his resignation amid admissions of drug use, alcohol addiction and various instances of being caught on tape making offensive comments.