[exim] Unexplained Long Retry Time

Exim 4.76

I have a recurring situation where emails sit in the queue for long periods of time for no apparent reason.

In a recent example, a message was received for multiple users.

This triggered the 'no immediate delivery: more than 10 messages...' exception and all the messages (13) got queued.

The queue runner is set to run every 15 minutes (-q15m)

Using the default retry rule.

Of the 13 messages, 6 were delivered in the next queue run (15:22). The 6 other messages were delayed various amounts (1-2 hours) and the example message three hours.

I don't know why the message wasn't delivered. A message was successfully delivered to the same user at 15:09:33 (30 seconds earlier)

2016-03-17 15:09:54 [20151] 1agdIv-0005F1-4x <= ... received
2016-03-17 15:09:54 [20151] 1agdIv-0005F1-4x no immediate delivery: more than 10 messages received in one connection
2016-03-17 15:22:55 [25736] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 15:37:36 [28488] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 15:52:31 [31281] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 16:08:55 [1908] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 16:25:00 [4726] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 16:37:33 [7125] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 16:52:33 [9571] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 17:14:43 [12099] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 17:22:47 [12977] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 17:37:55 [14470] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 17:54:41 [15611] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 18:10:17 [16755] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 18:22:33 [17615] 1agdIv-0005F1-4x == user@??? routing defer (-51): retry time not reached
2016-03-17 18:37:32 [18650] 1agdIv-0005F1-4x => ... delivered to local mailbox
2016-03-17 18:37:33 [18650] 1agdIv-0005F1-4x => ... delivered to gmail account
2016-03-17 18:37:33 [18650] 1agdIv-0005F1-4x Completed QT=3h27m40s

Thanks