Mail App Confounding Messages!

Hey folks!

Every now and then (it's becoming more common, actually) Mail will note that a new message has come in, but when I click on the new message, I find that it has replaced the new message with an old message from my Inbox (or a folder). In order to read the actual new message, I have to go online to my email provider to read it. Really. Annoying.
Does anyone have any suggestions as to how to fix this?

Thanks in advance!

 

Really? No thoughts? Argh...it's unbelievably annoying...alas.

 

I use Fastmail.Fm...but I'm pretty sure the problem is with Apple's Mail app, as when I go online I'm able to view the message as it's meant to be. It's weird...it has been working properly for years...and I haven't changed anything...