In case exim can not deliver a mail, it informs the sender by sending a warning message. To send out those messages only after a mail could not be delivered for 15 days, add the line

delay_warning = 15d

somewhere close to

timeout_frozen_after = ... 

in exims configuration file. The latter does - at least on debian systems - already exist. On Debian systems, there is no exim4.conf file. You have to edit exim4.conf.template instead and run update-exim2.conf afterwards.

exim4 -Mg <Message-Id>