Mail sits in outbox and doesn't send

I suddenly have a problem with mail sitting in my outbox and not sending on my iMac running iOS 10.10.4. My iPhone and iPad don't have this problem. It may be related to a recent iOS update, as it was;t having this problem before. This is occurring with all my mail accounts (yahoo, gmail, icloud, exchange) and not just one. All are green on the connection doctor, and all have the correct mail sender settings as well. Any thoughts?

Thanks.