====== Ubuntu - Email - SMTP Transaction ======

Simple SMTP dialogue

^ Client                                                                                                                                                                                                                                                                            ^ Server                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         ^
| Initiates a TCP connection to server.                                                                                                                                                                                                                                             | Presents an SMTP banner - that is, a greeting that starts with the code **220** to indicate that it is ready to speak SMTP (or usually ESMTP, a superset of SMTP):\\ <color /lightgrey>220 your.f.q.d.n ESTMP...</color>                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Introduces itself by way of an Hello command, either HELO (now obsolete) or EHLO, followed by its own Fully Qualified Domain Name:\\ <color /lightgrey>EHLO peers.f.q.d.n</color>                                                                                                 | Accepts this greeting with a **250** response. If the client used the extended version of the Hello command (**EHLO**), your server knows that it is capable of handling multi-line responses, and so will normally send back several lines indicating the capabilities offered by your server:\\ <color /lightgrey>\\ 250-your.f.q.d.n Hello ...\\ 250-SIZE 52428800\\ 250-8BITMIME\\ 250-PIPELINING\\ 250-STARTTLS\\ 250-AUTH\\ 250 HELP</color>\\ \\ If the **PIPELINING** capability is included in this response, the client can from this point forward issue several commands at once, without waiting for the response to each one.  |
| Starts a new mail transaction by specifying the Envelope Sender:\\ <color /lightgrey>MAIL FROM:<sender@address></color>                                                                                                                                                           | Issues a **250** response to indicate that the sender is accepted.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| Lists the Envelope Recipients of the message, one at a time, using the command:\\ <color /lightgrey>RCPT TO:<receiver@address></color>                                                                                                                                            | Issues a response to each command (**2xx**, **4xx**, or **5xx**, depending on whether delivery to this recipient was accepted, subject to a temporary failure, or rejected).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| Issues a **DATA** command to indicate that it is ready to send the message.                                                                                                                                                                                                       | Responds **354** to indicate that the command has been provisionally accepted.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| Transmits the message, starting with RFC 2822 compliant header lines (such as: From:, To:, Subject:, Date:, Message-ID:). The header and the body are separated by an empty line. To indicate the end of the message, the client sends a single period (".") on a separate line.  | Replies **250** to indicate that the message has been accepted.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| If there are more messages to be delivered, issues the next **MAIL FROM:** command.  Otherwise, it says **QUIT**, or in rare cases, simply disconnects.                                                                                                                           | Disconnects.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |