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Apols for the delay - at my age, sleep tends to be dominant!
Back to reality - but before I take up your latest suggestions, can you tell me the meaning of 2 clickables on my PC, sitting immediately above the list of emails, one marked with a small envelope and labelled Mark All As Read, the other, with a greyed out curved arrow labelled Undo. I took a chance and clicked on the first one which brought up a small window with "Mark 2,018 conversations as read. Are you sure you want to mark all the conversations as read?" (options OK or CANCEL). It appears at this point that the Undo button becomes live. That`s as far as I`ve gone for now. Does this make sense?