I don't mind IM's showing the 'read' bit, but for some reason I've always found read receipts on emails rude, and when it pops up the prompt to send the receipt

I don't mind IM's showing the 'read' bit, but for some reason I've always found read receipts on emails rude, and when it pops up the prompt to send the receipt back I always hit 'Cancel'. I wonder why.