ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt
-If Exim is built with EXPERIMENTAL_OCSP there is a new config
-boolean "prdr_enable" while controls whether PRDR is advertised
+If Exim is built with EXPERIMENTAL_PRDR there is a new config
+boolean "prdr_enable" which controls whether PRDR is advertised
as part of an EHLO response, a new "acl_data_smtp_prdr" ACL
(called for each recipient, after data arrives but before the
data ACL), and a new smtp transport option "hosts_try_prdr".