Unable to delete email

I have an IMac upgraded to El Capitan. I recently received an email from Bernie Sanders which I read and then put into trash. Mail will not let me delete the email. It gives me the message:The file “180791.emlx” couldn’t be opened because there is no such file. Because of this I can't delete all in my trash but have to delete one at a time. I've tried moving it to a different mail file. It won't let me do that either. What is going on?