ach command and response in the protocol consists of a string of ASCII characters followed by the line feed character $($ASCII code $10).$ All commands and responses are case sensitive$.$

An interaction begins when a client sends a LOGIN command to the server $($where is replaced with the name of the user to log in$).$ Usernames must not include space $($ASCII code $32)$ characters. If a LOGIN command is sent with the username containing a space, the server should see this message as an error.

The server responds to a LOGIN command with the number of messages that are currently stored on the system for the user with the given username. Thus, if there are no messages for the user, the server will respond with $0.$

After this response, the client can choose to either COMPOSE a message to be sent to a user, READ a message that was previously sent to the logged$-$in user, or EXIT the system.