(string, 0-256 characters) Formatting string for permanent delivery failure notifications. The default value of this option is the string:
Unable to deliver your message to $a; no further delivery attempts will be made.
To inhibit conversion of failure notifications, specify an empty string for this option. Note that GATEWAY_NOTIFICATIONS must be set to 1 for this option to be in effect. This option is ignored when GATEWAY_NOTIFICATIONS=0.