A channel program may be unable to deliver a message due to long-term service failures or invalid addresses. When this happens the PMDF channel program returns the message to the sender with an accompanying explanation of why the message was not delivered. PMDF will also optionally send a copy of all failed messages to the local postmaster. This is useful for monitoring message failures, but it can result in lots of traffic for the postmaster to deal with.
nosendpost are used to
control the sending of failed messages to the postmaster.
sendpost tells PMDF to send a copy of all failed messages
to the postmaster unconditionally.
PMDF to send a copy of the failure notice to the postmaster unless the
originator address on the failing message is blank; i.e., the
postmaster gets copies of all failed messages except those messages
that are actually themselves bounces or notifications.
errsendpost instructs PMDF to only send a copy of the
failure notice to the postmaster when the notice cannot be returned to
the originator. No failed messages are ever sent to the postmaster if
nosendpost is specified. The default if none of these
keywords is specified is to send a copy of failed mail messages to the
postmaster unless error returns are completely suppressed with a blank
Errors-to: header or a blank envelope From: address. Note that this
default behavior does not correspond to any of the keyword settings.