Chapter 2. Mail

The mail facility provides a method for exchanging electronic mail with users on the same system or on multiple systems connected by a network. This section documents the mail system and the standard mail user interface.

The mail facility provides a method for exchanging electronic mail (e-mail) with users on the same system or on multiple systems connected by a network. This section documents the mail system, the standard mail user interface, the Internet Message Access Protocol (IMAP), and the Post Office Protocol (POP).

The mail system is an internetwork mail delivery facility that consists of a user interface, a message routing program, and a message delivery program (or mailer). The mail system relays messages from one user to another on the same host, between hosts, and across network boundaries. It also performs a limited amount of message-header editing to put the message into a format that is appropriate for the receiving host.

A mail user interface enables users to create and send messages to, and receive messages from, other users. The mail system provides two user interfaces, mail and mhmail. The mail command is the standard mail user interface available on all UNIX systems. The mhmail command is the Message Handler (MH) user interface, an enhanced mail user interface designed for experienced users.

A message routing program routes messages to their destinations. The mail system's message routing program is the sendmail command, which is part of the Base Operating System (BOS) and is installed with BOS. The sendmail command is a daemon that uses information in the /etc/sendmail.cf file, the /etc/aliases file, and the /etc/sendmail.nl file to perform the necessary routing.

Depending on the type of route to the destination, the sendmail command uses different mailers to deliver messages (see figure).

As the figure illustrates:

• To deliver local mail, the sendmail program routes messages to the bellmail program. The bellmail program delivers all local mail by appending messages to the user's system mailbox, which is in the /varspool/mail directory.
• To deliver mail over a UNIX-to-UNIX Copy Program (UUCP) link, the sendmail program routes messages using Basic Network Utilities (BNU).
• To deliver Transmission Control Protocol/Internet Protocol (TCP/IP)-routed mail, the sendmail command establishes a TCP/IP connection to the remote system then uses Simple Mail Transfer Protocol (SMTP) to transfer the message to the remote system.

Mail Management Tasks

The following is a list of the tasks for which you, the mail manager, are responsible.

1. Configure the /etc/rc.tcpip file so that the sendmail daemon will be started at system boot time. See the instructions immediately following this list.
2. Customize the configuration file /etc/sendmail.cf. The default /etc/sendmail.cf file is configured so that local mail can be delivered. In order to deliver mail through TCP/IP or BNU, you must customize and compile the /etc/sendmail.cf file. See the "sendmail.cf File" in AIX Version 4.3 Files Reference for more information.
3. Define systemwide and domainwide mail aliases in the /etc/aliases file. See "Managing Mail Aliases" for more information.
4. Manage the Mail Queue. See "Managing the Mail Queue Files and Directories" for more information.
5. Manage the Mail Log. See "Managing Mail Logging" for more information.

Configuring the /etc/rc.tcpip File to Start the sendmail Daemon

To configure the /etc/rc.tcpip file so that the sendmail daemon will be started at system boot time:

1. Edit the /etc/rc.tcpip file with your favorite editor.
2. Find the line that begins with start /usr/lib/sendmail. By default, this line should be uncommented, that is, there is no # (pound sign) at the beginning of the line. However, if it is commented, delete the pound sign.
3. Save the file.

With this change, the system will start the sendmail daemon at boot time.