[exim] The delayed message has now been sent

Users assume that emails get passed on instantaneously and they are
often surprised (and concerned) to note from the mail server that a
particular mailed item is stuck in the outgoing queue. Once aware of a
delay, the user(s) would like a follow-up message from the mail server
to inform them that the delayed item has been delivered, i.e. if the
delivered message was delayed and the sender was informed, email the
sender to say that the message has now been delivered.

Can someone please point me to the appropriate Exim setting (or a
script)?

Thanks

John Woollatt