SMTP is used to pass
Email messages between
Internet Servers. Each message has a standardized header that is used to identify
Email address(es) of the person(s) the message is to be sent to, the
Email address and name of the sender (to whom responses can be sent automatically), and details of those nodes on the network through which the message passed. A number of extensions to SMTP have been defined yielding
ESMTP, which is mostly in use today.