The Mail System

The Mail System

The mail system is a group of programs that you can use to send messages to and receive messages from other users on the network. You can send mail through either UUCP or TCP/IP. The IRIX operating system uses MediaMail, System V /bin/mail, 4.3BSD /usr/sbin/Mail, and sendmail for its mail implementation.

The process of delivering mail involves four elements:

User Interface: The user interface creates new messages and reads, removes, and archives received messages. MediaMail, System V /bin/mail, and 4.3BSD /usr/sbin/Mail are the user interfaces provided with IRIX. Reference pages are available to fully describe the features of these interfaces, and MediaMail has an extensive online help system.
Mail Routing: A mail router examines each message and routes it through the network to the appropriate station. The sendmail program not only routes messages, but also formats them appropriately for their recipient stations.
Mail Transfer: A mail transfer program transmits messages from one station to another. sendmail implements the Simple Mail Transfer Protocol (SMTP) over TCP/IP. For TCP/IP mail, sendmail acts as an integrated routing and transfer program. In all cases, mail transfer has a counterpart: mail reception. In most cases, a single program provides both functions. UUCP is a mail transfer program that uses its own protocols and runs over serial lines.
Mail Delivery: A mail delivery program deposits mail into a data file for later perusal by a user or another program. The /bin/mail -d program delivers local mail.

After you compose a message by using MediaMail, /bin/mail, or /usr/sbin/Mail, the message is sent to sendmail, which attempts to determine the destination of the message. sendmail either calls /bin/mail (for mail to a user on the local station) or passes the message to the appropriate mail transfer program (for mail to a user on a remote station).

When sendmail receives a message from another station, it analyzes the recipient address; then, it either calls /bin/mail to complete the delivery if the local station is acting as a relay, or passes the message to the mail transfer program. For TCP/IP SMTP, sendmail also performs the mail transfer.

When you send a mail message on a network that uses TCP/IP, several layers of network software are involved. Figure 8-1 shows the layers of TCP/IP mail network software.

Figure 8-1 : Layers of TCP/IP Mail Software