What is typically the first command sent by a client to an SMTP server?

The first command sent by a client to an SMTP server is the EHLO command. This command is used to identify the client to the server and initiate the SMTP session. When the client sends EHLO, it typically includes its domain name, signaling to the server that the client is ready to communicate.

The EHLO command also plays a critical role in establishing the capabilities of the server. After receiving the EHLO command, the SMTP server responds with a list of supported extensions, which can include features like authentication and encryption methods. This exchange establishes the groundwork for what the client and server can do during their interaction, making it essential for proper communication.

Other commands, such as QUIT, DATA, and AUTH, serve distinct purposes within the SMTP protocol but are used at different points in the communication process. QUIT is sent when the client wishes to end the session, DATA is used to transfer email content after the mail transaction has started, and AUTH is typically employed after EHLO if the client needs to authenticate itself. Therefore, the sequence begins with EHLO to set the stage for the entire email sending process.