When the ISC2 mail server sends mail to other mail servers, it becomes a —?

When a mail server sends email to another mail server, it acts as anSMTP client. In the Simple Mail Transfer Protocol (SMTP), roles are defined by behavior, not by the system’s primary function.

The sending system initiates the connection and issues SMTP commands, which makes it the client for that transaction. The receiving mail server listens for incoming connections and acts as the SMTP server.

Mail servers routinely switch roles depending on the direction of communication. A single system may act as an SMTP server when receiving mail and as an SMTP client when sending mail.

Understanding SMTP roles is important for configuring mail security controls such as firewalls, TLS enforcement, spam filtering, and authentication. Security professionals must recognize that “client” and “server” are session-based roles, not permanent system identities.