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I am using Norton Internet Security v18.6.0.29 ....Since I've used NIS I very occasionally received this message in a Norton pop-up window when I send an email. Now it is happening several times a week...5 time just today. Each time I have to resend the email.Sometimes I receive it again. I didn't find anything understandable in general internet searching.  Can someone explain in simple terms what causes this message to be created and what I can do about it? Thx!