TIRED OF SLOW RESPONSE IN YAHOO MAIL SO MOVING TO GMAIL
SLOW RESPONSE IN MAIL UNREAD MESSAGES
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Delivery records in the full headers show when each computer received the message. The first delivery is at the bottom; the newest at the top. If you find a large time gap between delivery records, that shows which computer delayed before sending it to the next computer. https://help.yahoo.com/kb/SLN22026.html
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Nikolas Greek commented
VERY SLOW RESPONSE UNREAD MESSAGES