Here's the reply I got from the MCI people; I suspect others got it too,
but I'm reposting it here just in case.....
 
John Unsworth
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Date: Mon, 03 Aug 92 20:21:38 -0400
From: [log in to unmask]
 
MCI Mail has recently added some new security enhancements for
processing of electronic messages.  The implementation of these
features inadvertently caused some messages to be rejected that
were sent from the experimental Internet gateway to MCI Mail.  MCI
has made the appropriate system changes to rectify the situation
which will prevent this problem from reoccurring.
 
Our records indicate that you received a non-delivery notification
prior to implementing our system changes.  We apologize for any
inconvenience this has caused and request that you please resend
those messages for delivery.