By default, all messages queued to a channel are stored as files in the directory
/imta/queue/channel-name, where channel-name is the name of the channel. However, a channel that handles a large number of messages and tends to build up a large store of message files waiting for processing, for example, a TCP/IP channel, may get better performance out of the file system if those message files are spread across a number of subdirectories. The subdirs channel keyword provides this capability: it should be followed by an integer that specifies the number of subdirectories across which to spread messages for the channel, for example, tcp_local single_sys smtp subdirs 10.