Highlighted how? new messages has a yellow asterix beside then and the are bold.

Note here that if you have mail going to a phone or other mobile device that it may well be coming in as "read" hence flicking from unread to read immediately after download. That is bold to normal text.

I do use an I-Pad but this has just suddenly occurred, long after I began to use the I-Pad. Also, my wife has a separate address on the same account and her incoming emails are still highlighted.