SMTP servers enqueue messages, but so can a number of other channels including, the conversion channel and reprocess channel. A number of tasks are achieved during this phase of delivery, but the primary tasks are:
Running the addresses through the rewrite rules which do two things:
Rewrite the domain part of addresses into a desired format.
Direct messages to the appropriate channel queue.
The channel is the fundamental MTA component used for message processing. A channel represents a message connection with another system (for example, another MTA, another channel, or the local message store). As mail comes in, different messages require different routing and processing depending on the message’s source and destination. For example, mail to be delivered to a local message store will be processed differently from mail to be delivered to the internet which will be processed differently from mail to be sent to another MTA within the mail system. Channels provide the mechanism for customizing the processing and routing required for each connection. In a default installation, the majority of messages go to a channels handling internet, intranet, and local messages.
Specialized channels for specific situations can also be created. For example, suppose that a certain internet domain processes mail very slowly causing mail addressed to this domain to clog up the MTA. A special channel could be created to provide special handling for messages addressed to the slow domain, thus relieving the system of this domain bottleneck.
The domain part of the address determines to what channel the message is enqueued. The mechanism for reading the domain and determining the appropriate channel is called the rewrite rules (see 8.4 Rewrite Rules).
Channels typically consist of a channel queue and a channel processing program called a master program. After the slave program delivers the message to the appropriate channel queue, the master program performs the desired processing and routing. Channels, like rewrite rules, are specified and configured in the imta.cnf file. An example of a channel entry is shown below:
tcp_intranet smtp mx single_sys subdirs 20 noreverse maxjobs 7 SMTP_POOL maytlsserver allowswitchchannel saslswitchchannel tcp_auth tcp_intranet-daemon
The first word, in this case tcp_intranet is the channel name. The last word is called the channel tag. The words in between are called channel keywords and specify how messages are to be processed. Hundreds of different keywords allow messages to be processed in many ways. A complete description of channel keywords is provided in Chapter 12, Configuring Channel Definitions.
After the message is processed, the master program sends the message to the next stop along the message’s delivery path. This may be the intended recipient’s mailbox, another MTA, or even a different channel. Forwarding to another channel is not shown in the picture, but is common occurrence.