-PH/21 Added another message to those in 4.51/PH/42, namely "All relevant MX
- records point to non-existent hosts".
+PH/21 Added some other messages to those in 4.51/PH/42, namely "All relevant MX
+ records point to non-existent hosts", "retry timeout exceeded", and
+ "retry time not reached for any host after a long failure period".