SMTP Protocol overview

SMTP protocol is based on simple text-like commands. Client communicates with a server via these text commands. The sender as well as server-transmitter can initialize the session. When connection is established (via TCP usually) new session is opened. Three main commands have been used:

1. MAIL — specifies sender's address
   Example: MAIL FROM: <sender@domain.com>
2. RCPT — specifies recipient's address (can be used several times, each time for the new recipient)
   Example: RCPT TO: <receiver@domain.com>
3. DATA — specifies that the message data are coming
   Example: DATA message
4. SIZE — the size command tells the remote sendmail system the size of the attached message in bytes.
   Example: SIZE=numberofbytes
5. QUIT — this terminates an SMTP connection.
6. VRFY — this command will request that the receiving SMTP server verify that a given email username is valid.