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A mailhost is a system that you designate as the main mail system on your network. The mail-host is the system to which other systems at the site forward mail that they cannot deliver. You designate a system as a mailhost by adding the word mailhost to the Aliases line for the system entry in the Hosts database or by adding the word mailhost to the Internet Protocol (IP) address line in the system's /etc/hosts file. You should also use the file as the mail configuration file on the mailhost system.

A good candidate for a mailhost is a system that is attached to an Ethernet and to phone lines or a system configured as a router to the Internet. If you have a standalone system that is not networked but is in a time-sharing configuration, you can treat the standalone as the mailhost of a one-system network. Similarly, if you have several systems on an Ethernet and none have phones, you can designate one as the mailhost.Mail Server

A mail server is any system that stores mailboxes in the /var/mail directory. The mail server is responsible for routing all of the mail from a client. When a client sends mail, the mail server puts it in a queue for delivery. Once the mail is in the queue, the client can reboot or turn off the system without losing the mail messages. When the recipient gets mail from a client, the path in the From: line of the message contains the name of the mail server. If the recipient chooses to respond, the response goes to the user's mailbox on the server.

When the mailbox is on a mail server, messages are delivered to the server, not directly to the client's system. When the mailbox is on the user's local system, the local system acts as its own mail server if it does not mount /var/mail from another system.

If the mail server is not the user's local system, users with NFS can mount the /var/mail directory in the /etc/vfstab file, use the automounter, or log into the server to read their mail.

NOTE:  If you automount the /var/mail directory, you may have problems with mail on heterogeneous networks that have SunOS 4.x mail clients.

Good candidates for mail servers are systems that provide a home directory for users or that are backed up on a regular basis.

Table 1-1 shows some sample statistics about the size of mail messages and mail traffic at a computer company with about 12,000 employees.

Table 1-1 Sample Statistics for Mail Messages and Traffic

Statistic Description
6,500 bytes Average size of an email message
140 kilobytes Amount of mail received by an average user in one day
15 kilobytes Small mailbox size (user reads mail regularly and stores messages elsewhere)
40 megabytes Large mailbox size (user stores long-term mail in /var/mail mailbox)
18,000 messages Average number of messages per day sent outside of the company
55,000 messages Average number of messages per day received from outside of the company
2 megabytes Recommended spooling space to allocate for each user's mailbox, based on the figures in this table

NOTE:  The information in Table1-1 is valid for ASCII messages only. With the advent of multimedia mail, which lets users transmit any type of data (not just ASCII text), the average size of an email message is likely to grow enormously. In the future, system administrators will need to allocate more spooling space for multimedia mailboxes.

Mail Client

A mail client is any system that receives mail on a mail server and does not have a local /var/mail directory, but instead mounts /var/mail using NFS. You must make sure the mail client has the appropriate entry in the /etc/vfstab file and a mount point to mount the mailbox from the mail server.

User Agent

The user agent is the program that acts as the interface between the user and the sendmail program. The user agents for SunOS 5.x system software are /usr/bin/mail, /usr/bin/mailx, $OPENWINHOME/bin/mailtool, and /usr/dt/bin/dtmail.

Mail Transport Agent

The transport agent is responsible for actually receiving and delivering messages. The transport agent for SunOS 5.x system software is sendmail. The transport agent performs the following functions:

  Accepts messages from the user agent
  Understands destination addresses
  Delivers mail originating on the local system to the proper mailbox(es) if local, or to a delivery agent if not local
  Receives incoming mail from other delivery agents and delivers it to local users

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