That’s not what the flags are for—they are merely a reminder to yourself, that helps you locate that particular email at a later time.

Yahoo doesn’t offer the "read receipt" on the free versions–altho, I understand that "Yahoo Plus" (which you have to pay for) does, by syncing with Outlook Ex.

The only draw back to the read receipts, is that it isn’t automatic. The person recieving the mail, MUST agree to send a receipt back to you—(which is a pain in the butt)–AND if they choose not to do that—then you still won’t know when/if they opened it.