If like Gmail, after you mark the sender as a spammer, all of the mail should wind up in your spam box. Go to your spam box and open the message. Then open the message.
There should be a button that says "Not Spam" ... I've done what you did before on Gmail and Yahoo and it was pretty easy to get it back to normal again.
But if your talking about Outlook or whatever Mac program, etc... good luck. I would expect it to be similar but am not sure as I don't bother with POP email programs just online emailing sites.