As of about 4 days ago, an imap account in Mail starting behaving more like a pop account - I can still send and receive mail, but the server isn

As of about 4 days ago, an imap account in Mail starting behaving more like a pop account - I can still send and receive mail, but the server isn't updated with what I've done on the Mac. It still works properly from another machine so it can't be a server issue.