### Introduction to Sendmail

Sendmail is the standard Unix program for mail transport and delivery. When a user on your system wants to send email to someone on another host, sendmail is the program that does the actual process of delivering the mail. Similarly, when someone on another system sends mail to one of your users, sendmail will receive the email and store it in the user's mail file.

Sendmail is an MTA (mail transport agent) only, meaning that it does not present any user interface to people wishing to send email. Programs like Eudora, elm or /bin/mail that users actually interact with a called MUAs (mail user agents). An MUA does not deliver email itself, instead it simply passes the email on to sendmail to do the delivery.

User MUAs can be run either directly on your system (MUAs like pine, elm or /bin/mail), or on a separate host which may be a single-user PC (for MUAs like Eudora, Exchange and Netscape Communicator). In the first case, mail is sent be directly invoking sendmail and passing the message to it, and received by reading the user's mail spool file. For MUAs run on other hosts, mail is sent by connecting to the sendmail process on your system using the SMTP protocol, and received by connecting to a separate POP3 server.

### The Sendmail Module

The main page of this module shows a table of icons, each for configuring a different part of sendmail's functionality. However, the features behind some icons may not be available, if they have not been setup in your `sendmail.cf`. When installing sendmail, you can choose which features are available by editing an `.mc`, which is converted by `m4` into your `sendmail.cf`. Many features (such as Address Mapping, Domain Masquerading and Spam Control) may not be available unless you have enabled them at install time.