My own sent emails being marked as junk

My mac insists on marking MY OWN sent email as junk. As in, emails I have just composed with my very own hands. The app doesn't put them into the junk folder — it just flags them with the yellow "Mail thinks this message is Junk Mail" banner. I'd guess that at least 75% of my sent mail from this address is routinely flagged as junk. Drives me bananas. It only happens with my Yahoo email address, and it doesn't happen in the browser version of Yahoo mail. Only in Apple Mail. The problematic address *is* in my Contacts app. I even have a filter in my Yahoo account that's supposed to keep all mail sent from that address out of junk. Doesn't work. Any hints??