Sendmail

SendmailTM implements a general purpose internetwork mail routing facility under the UNIX® operating
system. It is not tied to any one transport protocol — its function may be likened to a crossbar switch, relaying messages from one domain into another.

In the process, it can do a limited amount of message header editing to put the message into a format that is appropriate for the receiving domain. All of this is done under the control of a configuration file.

Due to the requirements of flexibility for sendmail, the configuration file can seem somewhat unapproachable.

However, there are only a few basic configurations for most sites, for which standard configuration files have been supplied. Most other configurations can be built by adjusting an existing configuration file incrementally.

Sendmail is based on: RFC 821 (Simple Mail Transport Protocol), RFC 822 (Internet Mail Headers Format), RFC 974 (MX routing), RFC 1123 (Internet Host Requirements), RFC 1413 (Identification server), RFC 1652 (SMTP 8BITMIME Extension), RFC 1869 (SMTP Service Extensions), RFC 1870 (SMTP SIZE Extension), RFC 1891 (SMTP Delivery Status Notifications), RFC 1892 (Multipart/Report), RFC 1893 (Enhanced Mail System Status Codes), RFC 1894 (Delivery Status Notifications), RFC 1985 (SMTP Service Extension for Remote Message Queue Starting), RFC 2033 (Local Message Transmission Protocol), RFC 2034 (SMTP Service Extension for Returning Enhanced Error Codes), RFC 2045 (MIME), RFC 2476 (Message Submission), RFC 2487 (SMTP Service Extension for Secure SMTP over TLS), RFC 2554 (SMTP Service Extension for Authentication), RFC 2821 (Simple Mail Transfer Protocol), RFC 2822 (Internet Message Format), RFC 2852 (Deliver By SMTP Service Extension), and RFC 2920 (SMTP Service Extension for Command Pipelining).

However, since sendmail is designed to work in a wider world, in many cases it can be configured to exceed these protocols.