Can't delete mac mail messages after el capitan update
I upgraded to El Cap yesterday, and mac mail has been behaving oddly ever since. The most obvious symptom is that it won't delete any messages. I delete them, then a while later I get a pop-up that says it was unable to move the "xxx" message to the trash - only response option is OK, so I click OK and the message reappears in my inbox.
Any suggestions?
MacBook Air (13-inch, Early 2014), OS X El Capitan (10.11)