Note: you should not enable this unless you are using Sendmail 8.8.x or greater.
This variable sets the request for when notification is returned. The string consists of a comma separated list (no spaces!) of one or more of the following: never, to never request notification, failure, to request notification on transmission failure, delay, to be notified of message delays, success, to be notified of successful transmission.
Example: set dsn_notify="failure,delay"