Chapter 5. SMTP AUTH and STARTTLS
To prevent a system from being a source of spam while still enabling legitimate users to send outbound messages, a secure mechanism must be provided to relay messages that are destined for email recipients located on remote hosts. In the past, relay restrictions were usually based upon IP addresses, which work fine for certain classes of users such as enterprise networks located on high-speed networks. However, the advent of roaming users created a need for dial-in users to send email from remote networks. As a result, a method of authenticating these mobile users had to be developed.
There several ways users can be authenticated prior to allowing them to send email to arbitrary email boxes: