Search for previously read messages

There is an easy way to search for unread messages in Apple Mail. But there needs to be another category for SORT FOR READ MESSAGES.

I would like to locate messages that I have previously opened.

My Apple mail shows there are >800 of them. How do I find them? I have emails for continuing education credits that I have previously opened and no matter what keyword I use, I can't find them. Help!

I have been using Macs for 30 years and cannot believe this is so elusive!

Anne