The MIME standard provides the message/partial content type for breaking up messages into smaller parts. This is useful when messages have to traverse networks with size limits. Information is included in each part so that the message can be automatically reassembled after it arrives at its destination.
The defragment channel keyword and the defragmentation channel provide the means to reassemble messages in IMTA. When a channel is marked defragment, any message or partial messages queued to the channel are placed in the defragmentation channel queue instead. After all the parts have arrived, the message is rebuilt and sent on its way. The nodefragment disables this special processing. nodefragment is the default.
A defragment channel must be added to the IMTA configuration file in order for the defragment keyword to have any effect. If your configuration was built by the IMTA configuration utility, then you should already have such a channel.