A brief reminder of the SMTP protocol
Spam

For a proper understanding of the techniques discussed in this file, a few brief reminders about e-mail exchanges are in order.

In an electronic message exchange architecture, there are two types of component (see figure 11):

• MTA (Mail Transfer Agent): server for routing emails in the transit network (e.g. ISP networks, corporate networks);

• MUA (Mail User Agent) – also called UA (User Agent) –: end-user mail client (e.g. Microsoft Outlook, Mozilla Thunderbird, etc.).

Note that the last MTA responsible for delivering the email to the MUA is also sometimes called the MDA (Mail Delivery Agent).

Dialogue between MTAs takes place using the SMTP protocol (RFC 2821). In concrete terms, the MTAs will exchange messages in the form of MAIL FROM, HELO......