Which command initiates the sending of an email in SMTP?

The command that initiates the sending of an email in SMTP is "MAIL FROM." This command is part of the SMTP protocol that specifies who is sending the email. When the sending client issues the MAIL FROM command, it informs the SMTP server about the email address of the sender, which sets the context for the subsequent commands that define the recipient, the message, and other relevant details.

This command is essential because it allows the server to understand the source of the email, which is crucial for processing and validating the sending email transaction. Following this command, other commands will typically specify the recipient and then proceed to the actual transmission of the email content.

The other commands mentioned serve different purposes. CONNECT typically refers to establishing a connection with the server, STARTTLS is used to initiate a secure connection using TLS (Transport Layer Security), and BEGIN is not a standard SMTP command used in the email sending process. Understanding these distinctions clarifies why "MAIL FROM" is the correct choice in this context.