SMTP (Simple Mail Transfer Protocol) is a communication protocol used to transport electronic mail (email) messages between servers. It is mainly responsible for the message transport aspect of email communication. SMTP defines the process by which email messages are sent from the sender's mail server to the recipient's mail server.

1. Message Transport:
SMTP is primarily designed to handle the transportation of email messages. It provides a set of rules and guidelines that enable the reliable and efficient transfer of email between mail servers. When a user sends an email, the SMTP client on their computer establishes a connection with the SMTP server of their email service provider. The client then transfers the email message to the server, which is responsible for forwarding it to the recipient's mail server.

2. Message Encryption:
SMTP itself does not provide encryption for email messages. However, there is a variant of SMTP called SMTPS (SMTP Secure) that uses Secure Sockets Layer (SSL) or Transport Layer Security (TLS) encryption to secure the communication between the email client and the server. SMTPS ensures that the email content cannot be intercepted or tampered with during transmission.

3. Message Content:
SMTP is not concerned with the content of the email messages being transmitted. It treats email messages as a stream of text, regardless of the format or content they contain. The actual content of the email, including the subject, body, attachments, and any other elements, is not specified or regulated by the SMTP protocol. The content is typically handled by other protocols such as MIME (Multipurpose Internet Mail Extensions).

In summary, SMTP is primarily responsible for the message transport aspect of email communication. It defines the rules and procedures for reliably transferring email messages between mail servers. SMTP does not handle message encryption or define the content of the email itself.