# Telnet SMTP

The sending and communication are also done by special commands that cause the SMTP server to do what the user requires.

| **Command**  | **Description**                                                                                  |
| ------------ | ------------------------------------------------------------------------------------------------ |
| `AUTH PLAIN` | AUTH is a service extension used to authenticate the client.                                     |
| `HELO`       | The client logs in with its computer name and thus starts the session.                           |
| `MAIL FROM`  | The client names the email sender.                                                               |
| `RCPT TO`    | The client names the email recipient.                                                            |
| `DATA`       | The client initiates the transmission of the email.                                              |
| `RSET`       | The client aborts the initiated transmission but keeps the connection between client and server. |
| `VRFY`       | The client checks if a mailbox is available for message transfer.                                |
| `EXPN`       | The client also checks if a mailbox is available for messaging with this command.                |
| `NOOP`       | The client requests a response from the server to prevent disconnection due to time-out.         |
| `QUIT`       | The client terminates the session.                                                               |