+9. When an address is delayed because of a 4xx response to a RCPT command, it
+ is now the combination of sender and recipient that is delayed in subsequent
+ queue runs until its retry time is reached. You can revert to the previous
+ behavious, that is, delay the recipient independent of the sender, by
+ setting address_retry_include_sender=false in the smtp transport. However,
+ this can lead to problems with servers that regularly issue 4xx responses to
+ RCPT commands.
+
+