The email address that receives notification messages for every message that has had delivery errors.
If it is empty (allowed), the notification message is sent to the sender only.
If this variable is specified and is not empty, its content is sent as HELO domain.
Otherwise the reverse lookup of the local IP is sent as HELO domain.
This helps to deal with remote SMTP servers that are set to check the reverse lookup
of the incoming IP.
List of delivery attempts that require the system to send a notification to the sender
(and eventually to 'TempErrorsAdmin').
The list is a comma separated list of numbers (with no extra spaces) as in:
"1,4,9"
Default is empty which means no notification is sent upon a delivery attempt failure.