As described in the previous section, the delivery of an email message begins with the establishment of an SMTP session between the devices sending and receiving the message. The SMTP sender initiates a TCP connection to the SMTP receiver and then sends a HELO or an EHLO command, to which the receiver responds. Assuming no problems ensue, the session is then established and ready for actual email message transactions.
The SMTP mail transaction process itself consists of three steps:
Transaction Initiation and Sender Identification The SMTP sender tells the SMTP receiver that it wants to start sending a message and gives the receiver the email address of the message's originator.