What is SMTP?
SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol that can be used to send and receive e-mails. However, SMTP is limited in its ability to process messages on the receiving end. Hence it is usually done with either of the POP3 or IMAP protocols. These protocols save messages in a server mailbox and fetch them from the server at periodic intervals. They often use a program that uses SMTP to send e-mails and POP3 or IMAP to receive them.
On UNIX-based systems, Sendmail and Postfix are mostly used as SMTP servers for e-mails. Postfix has grown in popularity in recent years because it is easier to configure and it is more modular. In the commercial version, Sendmail already includes a POP3 server. Finally, Microsoft Exchange also offers an SMTP server and can be configured to support POP3.
SMTP is usually implemented in such a way that it operates over port 25 on the Internet. An alternative to SMTP is X.400. It is mainly used in Europe and Canada. Many mail servers now support ESMTP (Extended Simple Mail Transfer Protocol), which allows multimedia files to be sent with e-mails.
SMTP has two disadvantages that are inherent in the network protocol. The first is that sending an email via SMTP does not return any confirmation of dispatch. Although the specifications of the protocol provide for such a notification, its formatting is not specified by default, so that usually only an English-language error message including the header of the undelivered message is returned. This makes it difficult to determine the cause of the failed transmission (e.g. an incorrect address or an overfilled mailbox at the recipient).
The second disadvantage of SMTP is that users are not authenticated when a connection is established, and the sender of an e-mail is therefore not reliable. As a result, open SMTP relays are often misused to send mass spam. There are many security techniques in use today to prevent the misuse of SMTP servers. For example, suspicious e-mails are rejected or moved to quarantine (spam folder).