Just a thought Bill - some of your emails may be returned, not because of a full email 'Inbox' but due to Spam rejecting programs on the receiving computers. Depending on how the program is configured, it may reject emails which do not contain the actual email address of the receiver. I use 'MailWasher', which automatically marks 'not to me' emails, for bouncing. Your KN file emails of course do not contain specific addresses and unless they are manually marked as 'Friend' using options within the program they could be treated as Spam and therefore bounced.
I realise that it would not be practical for you to send to individual addressees, so it is up to others, if using Spam Rejection to make sure that their program is correctly configured, if they wish to receive the KN files.

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Willum