SMTP is a text-based TCP protocol that works on port 25. SMTP works in a lock-step, one-at-a-time dialog, with the client sending commands and the server sending responses for each command.

In a typical session, the dialog goes as follows:

1. The client first establishes a connection to the SMTP server.
2. The server initiates with a greeting. This greeting indicates that the server is ready to receive commands.
3. The client then issues its own greeting.
4. The server responds.
5. The client sends a command indicating who the mail is from.
6. The server responds to indicate that the sender is accepted.
7. The client issues another command, which specifies the mail's recipient.
8. The server responds indicating the recipient is accepted.
9. The client then issues a DATA command.
10. The server responds asking the client to proceed.
11. The client transfers the email.

The protocol is...