Final answer:
In SMTP, a single dot on a line by itself marks the end of a message body, signaling the SMTP server that the client has finished sending the body of the message.
Step-by-step explanation:
In the Simple Mail Transfer Protocol (SMTP), the end of a message body is marked by a line containing only a period (single dot), which is also referred to as the dot character (.). This is sent on a line by itself and is used to indicate to the SMTP server that all the data for the current mail transaction has been sent. The SMTP client sends this sequence to inform the server that it has completed sending the body of the message, and it is now expecting the SMTP server's response indicating the acceptance, rejection, or a continuation request for further data.