A channel is an interface with another SIMS component, another email system, or mail user agent. The actual hardware connection or software transport or both may vary widely from one channel to the next.
Each channel consists of up to two channel programs and an outgoing message queue for storing messages that are destined to be sent to one or more of the interfaces associated with the channel. Channel programs perform two functions:
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They transmit messages to other interfaces, deleting them from their queue after they are sent. |
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They accept messages from other interfaces, placing them or enqueueing them into channel queues. Note that while a channel program only removes messages from its own queue, it can enqueue messages in any queue, including its own. |
A channel program that initiates a message transfer to another interface on its own is called a master program. A program that accepts transfers initiated by another interface is called a slave program. A channel can be served by a master program, a slave program or both. Some of the default channels provided are:
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SMTP channel: TCP/IP-based message delivery and receipt. |
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Pipe channel: used for alternative message delivery programs. |
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Message Store channel: delivers mail to the Sun Message Store. |
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Reprocessing channel: used for messages that are resubmitted due to transient failures during delivery. |
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Defragmentation channel: reassembles partial messages into the original complete message. |
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Conversion channel: performs body part-by-body part conversion on messages. |
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Local channel: delivers mail to /var/mail for backward compatibility. |
SMTP channels differ according to the set of SMTP hosts with which they communicate. One type of SMTP channel is dedicated to relaying mail to and from one single particular host. For example, if the IMTA is installed within a Firewall, all mail addressed to external users must transit through the Firewall host. Therefore, an SMTP channel must be configured to handle messages between the IMTA and this Firewall host. This type of channel is referred to as an SMTP router channel.
A second type of SMTP channel is dedicated to relaying messages between the IMTA and a group of SMTP hosts defined by the knowledge of the DNS. In this type of channel, envelope recipient addresses are used to determine the destination host(s) of the message. This type of channel is called SMTP intranet or SMTP internet channel, depending on whether they are used to exchange mail with hosts inside or outside of your mail network.
The local channel has two purposes. The main purpose is to determine the delivery options of local users. The other purpose is to deliver mail to Solaris Operating Environment mailboxes (/var/mail files).
The message store channel is used to deliver messages to the Sun Message Store.
The pipe channel can be used to deliver messages through programs provided and configured by the site administrator (alternative message delivery programs). The pipe channel can be used to implement interfaces with other systems/components that are not provided with SIMS to invoke mail filtering programs, or mail auto-responders/auto-forwarders.
SIMS currently uses the pipe channel to implement autoreply-based functions such as the vacation utility.Refer to "Alternative Delivery Programs" on page 102 for information on how to make delivery programs available to users.