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 timeouts can occur, which in turn can lead to repeated message submission attempts and other problems.
IMTA 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 online delays enormously. Note, however, that the processing overhead is only deferred, not avoided completely.
This special facility is activated by using a combination of 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 forces 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 reprocessing channel is used to perform the deferred processing and must be added to the configuration file in order for the expandlimit keyword to have any effect. If your configuration was built by the IMTA configuration utility, then you should already have such a channel.