+PH/19 When calculating a retry time, Exim used to measure the "time since
+ failure" by looking at the "first failed" field in the retry record. Now
+ it does not use this if it is later than than the arrival time of the
+ message. Instead it uses the arrival time. This makes for better
+ behaviour in cases where some deliveries succeed, thus re-setting the
+ "first failed" field. An example is a quota failure for a huge message
+ when small messages continue to be delivered. Without this change, the
+ "time since failure" will always be short, possible causing more frequent
+ delivery attempts for the huge message than are intended.
+