Like the SMTP client, the POP3 client uses a series of four-character commands for communicating with the server. The server responds with a very small number of alphabetic responses, such as +OK (indicating that the command was executed) and -ERR (indicating that the command resulted in an error). The responses might also include additional arguments or parameters. Each message in the mailbox is referenced by a message number. The client sends a RETR (retrieve) command to the server to download a message. The DELE command deletes a message from the server.