In addition to returning messages, the MTA can send detailed warnings for undelivered messages. This is generally due to time-outs based on the setting of the notices channel keyword, although in some cases channel programs may produce warning messages after failed delivery attempts. The warning messages contain a description of what’s wrong and how long delivery attempts continue. In most cases they also contain the headers and the first few lines of the message in question.
Optionally, a copy of all warning messages can be sent to the local postmaster. This can be somewhat useful for monitoring the state of the various queues, although it does result in lots of traffic for the postmaster to deal with. The keywords warnpost, copywarnpost, errwarnpost, and nowarnpost are used to control the sending of warning messages to the postmaster. (See Table 10–12.)