To get a speeding ticket issued from a police officer the officer would pull you over and let you know they witnessed you speeding and then the police officer will issue you the ticket after pulling you over.
The police would not be able to just mail you a ticket after witnessing speeding as they would have to give you the ticket as soon as they noticed you speeding.
There are traffic cameras though that take your license plate number down and then they can issue you a speeding ticket or red light ticket or other traffic ticket for violations witnessed on the cameras.
But you won't get a speeding ticket from an officer directly without them pulling you over first.