Messages to a handful of domains fail to be delivered.  The logging was
turned up via regedit to level 7.  There are no events that indicate a
message was not sent, and there are no messages stuck in any queue.
The majority of messages are sent without issue, and the server can ...

We have recently started to experience an intermittent email delivery
problem. Users complain when they receive and NDR to the tune of:
The following recipient(s) could not
be reached: