Most 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 (online delays). If the delays are long enough, network time-outs can occur, which in turn can lead to repeated message submission attempts and other problems.
The MTA provides a special facility to force deferred (offline) processing if more than a given number of addresses are specified for a single message. 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 expandlimit keyword. The expandlimit keyword takes an integer argument that specifies how many addresses should be accepted in messages coming from the channel before deferring processing. The default value is infinite if the expandlimit keyword is not specified. A value of 0 will force deferred processing on all incoming addresses from the channel.
The expandlimit keyword 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 using the expandchannel keyword; the reprocessing channel is used by default, if expandchannel is not specified, but use of some other reprocessing or processing channel may be useful for special purposes. If a channel for deferred processing is specified via expandchannel, 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 expandlimit keyword to have any effect. If your configuration was built by the MTA configuration utility, then you should already have a reprocessing channel.
Extraordinarily large lists of recipient addresses are often a characteristic of unsolicited bulk email. The holdlimit keyword tells the MTA that messages coming in the channel that result in more than the specified number of recipients should be marked as .HELD messages and enqueued to the reprocess channel (or to whatever channel is specified via the expandchannel keyword). The files will sit unprocessed in the reprocess queue awaiting manual intervention by the MTA postmaster.