In order to diagnose and repair PMDF message delivery problems, you need some understanding of the steps that PMDF goes through to process and deliver a message. Each time a message is handed to PMDF, a file is created in the appropriate message queue subdirectory. Each channel has a corresponding subdirectory. The subdirectory into which the message is placed is that of the destination channel which will handle the message next. For example, a message coming in from any source to be delivered to a local user will be placed in the subdirectory corresponding to the local, or L, channel. A message coming in from UUCP to be routed on through SMTP over TCP will be placed in the subdirectory for the outbound TCP/IP channel, such as tcp_local.

Once the message is stored in the appropriate queue subdirectory, a PMDF processing job is usually submitted to the PMDF Job Controller. This step is omitted for channels marked with the periodic keyword. The processing job will attempt to deliver the message. If it cannot be delivered and the failure is judged to be a permanent condition, the message is returned to the sender and the local postmaster. If the failure is a temporary error condition, the message is left in the message queue to be retried later.

For additional discussion of PMDF operation, see also Chapter 1.