SMTP is the Simple Mail Transfer Protocol. It is the most popular protocol for transferring electronic mail on the Internet. SMTP is quite robust, but provides no facilities for Authentication of the sender or recipient; for encryption; or for attaching non-ASCII data.
Extensions to SMTP allow for each of the above features, however:
The most popular software for routing and managing SMTP mail is
Sendmail. Historically, this program has suffered from numerous
Security Exploit's. Modern versions of Sendmail, however, are
robust, and support effective defenses against Spam and unauthorized
Mail Relay use.