Net::SMTP operates at the lowest level of the e-mail access modules. It interacts directly with the SMTP daemons to transmit e-mail across the Internet. To use it effectively, you must know a bit about the innards of SMTP. The payoff for this added complexity is that Net::SMTP is completely portable, and works as well from Macintoshes and Windows machines as from UNIX systems.
The SMTP Protocol
When a client e-mail program wants to send mail, it opens a network connection to a mail server somewhere using the standard SMTP port, number 25. The client conducts a brief conversation with the server, during which time it establishes its identity, announces that it wishes to send mail to a certain party, and transmits the e-mail message. ...