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.