Programming Internet Email

Chapter 9. The Extended Simple Mail Transfer Protocol

In this chapter:

• Using ESMTP

• ESMTP Commands

• ESMTP Sessions

The Simple Mail Transfer Protocol and its more modern extensions form the backbone of the Internet Mail System. Since 1982, when SMTP was first formalized, the protocol has grown and changed considerably. Today, this simple protocol still forms the basis for Internet mail traffic. Chapter 1, Electronic Mail on the Internet, provided an overview of the Internet mail system and showed how SMTP was used by MTAs to deliver mail.

SMTP is used by MUAs to send mail messages to MTAs and by MTAs to transfer mail other MTAs. MTAs that provide TCP/IP-based SMTP service, such as sendmail, listen on port 25.

The many extensions to SMTP have become known as the Extended Simple Mail Transfer Protocol (ESMTP), or more formally as “SMTP with service extensions.” I will use the term"ESMTP” to refer to the basic protocol and whatever service extensions a given server chooses to support. The term “SMTP” will refer to the basic protocol without service extensions.

ESMTP is defined piecemeal in a large number of documents. This chapter is an attempt to provide a description and reference to the current state of ESTMP.

Internet Standard Number 10 is made up of three standard RFCs; RFC 821 (“SMTP”), RFC 1869 (“SMTP Service Extensions”), and RFC 1870 (“SMTP Service Extension for Message Size Declaration”). Together, these documents define not only the standard for transmitting Internet email, but also ...