I received notification that a subscriber had been dropped from my
mailing list today due to a "user not found" error.

My error monitoring is set up to drop subscribers after 100 errors or
errors for 4 days.

The only problem is that no previous errors for this user had been
reported.  Today's notification was the first indication that there
was any problem with this particular user.

The user has his option set for non-digest mode.  The mailing list has
handled about 35 messages during each of the past 4 days.  The only
possible explanation that I can see is that, for some reason, perhaps
his ISP returned multiple error messages for each undelivered mail message
yesterday.  This seems so unlikely that I wonder if anyone can provide
any other reason why the user was dropped so suddenly by the
error monitoring system.

And what do I tell this user if he asks why he was dropped?

                                               Les