The notices keyword controls the amount of time an undeliverable message is silently retained in a given channel queue. IMTA is capable of returning a series of warning messages to the originator and, if the message remains undeliverable, IMTA eventually returns the entire message.
The keyword is followed by a list of up to five monotonically increasing integer values. These values refer to the message ages at which warning messages are sent. The ages have units of days if the RETURN_UNITS option is 0 or not specified in the option file; or hours if the RETURN_UNITS option is 1. When an undeliverable message attains or exceeds the last listed age, it is returned (bounced).
When a message attains any of the other ages, a warning notice is sent. The default if no notices keyword is given is to use the notices setting for the local channel. If no setting has been made for the local channel, then the defaults 3, 6, 9, 12 are used, meaning that warning messages are sent when the message attains the ages 3, 6, and 9 days (or hours) and the message is returned after remaining in the channel queue for more than 12 days (or hours).
Note - The syntax for the notices keyword uses no punctuation. For example, the default return policy is expressed as: notices 3 6 9 12.
The following line specifies that if messages are enqueued to the tcp_local channel and deferred for later reprocessing, transient failure delivery status notifications will be generated after 1 and 2 days. If the message is still not delivered after 5 days, it will be returned to its originator.
The defaults channel appears immediately after the first blank line in the configuration file, usually /imta/table/imta.cnf. It is important that a blank line appear before and after the line defaults notices....