Expandlimit, expandchannel, holdlimit Channel Options

Expansion of multiple addresses
Most MTA channels support the specification of multiple recipient addresses in the transfer of each inbound message. The specification of many recipient addresses in a single message may result in delays in message transfer processing ("on-line" delays). In particular, multi-recipient messages that require a great deal of processing of the message body can be affected by processing delays, or that require creation of many different file copies on disk in the MTA queue area can be affected by slow disk performance. If the resulting delays are long enough, network timeouts can occur, which in turn can lead to repeated message submission attempts and other problems.

The MTA provides a special facility to force deferred ("off-line") processing of additional recipient message copies once a given number of addresses are specified for a single message. The deferral happens after processing of the "initial" recipients (those before the  value was reached), and after address processing for the additional recipients too, (for instance, after   mapping table  checks and after alias processing), but before message processing. In particular, such deferral means that for the "additional" recipients, only one message file (storing all of the "additional" recipients), is written to the queue area (to a  or    channel queue area, depending upon use of the   channel option). Deferral of message processing can decrease on-line delays enormously. Note, however, that the processing overhead is deferred, not avoided completely.

This special facility is activated by using a combination of, for instance, the generic reprocessing channel and the  channel option. The  option takes an integer argument that specifies at what number of recipients to begin deferring processing of the message copy (or copies) to that and additional recipient addresses. The default value is effectively infinite if the  channel option is not specified. A value of 1 will force deferred processing on all incoming addresses from the channel.

The  channel option must not be specified on the local channel or the reprocessing channel itself; the results of such a specification are unpredictable.

The channel actually used to perform the deferred processing may be specified on a per-source-channel basis using the  channel option; the reprocessing channel is used by default, if   is not specified, but use of some other reprocessing or processing channel may be useful for some purposes. In particular, for Messaging Server MTA versions 5.2 and earlier, typical configuration usage required that a processing channel, rather than a reprocessing channel, be used. If a channel for deferred processing is specified via, that channel should be a reprocessing or processing channel; specification of other sorts of channels may lead to unpredictable results.

The reprocessing channel, or whatever channel is used to perform the deferred processing, must be added to the MTA configuration file in order for the   option to have any effect. If your configuration was built by the initial configuration utility, then you should already have a reprocessing channel.

(Note that typical Messaging Server sites running version 5.2 or earler could not use the  option unless they also marked the affected channel   (or   redirecting the expansion to an alternate   sort of channel), as enqueues to a channel marked    would not succeed if deferred to a   channel.)

Extraordinarily large lists of recipient addresses are often a characteristic of so-called SPAM---junk e-mail. The  channel option tells the MTA that messages coming in the channel that result in more than the specified number of recipients should be marked as   messages and enqueued to the   channel (or to whatever channel is specified via the   channel option). As  messages, the files will sit unprocessed in that MTA queue area awaiting manual intervention by the MTA postmaster.

See also:
 * Recipient access mapping tables
 * Process and reprocess channels
 * viaaliasrequired Option
 * File creation in the MTA queue area channel options
 * Processing control and job submission channel options
 * Long address lists or headers channel options
 * Channel options