A channel program may be unable to deliver a message because of long-term service failures or invalid addresses. When this failure occurs, the IMTA channel program returns the message to the sender with an accompanying explanation of why the message was not delivered. Optionally, a copy of all failed messages is sent 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.
The keywords sendpost, copysendpost, errsendpost, and nosendpost control the sending of failed messages to the postmaster. sendpost tells IMTA to send a copy of all failed messages to the postmaster unconditionally. copysendpost instructs IMTA to send a copy of the failure notice to the postmaster unless the originator address on the failing message is blank, in which case, the postmaster gets copies of all failed messages except those messages that are actually themselves bounces or notifications.
errsendpost instructs IMTA to send a copy of the failure notice only 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.This default behavior does not correspond to any of the keyword settings.