Outlook keeps sending the same email repeatedly every ten minutes

I just installed Outlook in Office 2011 for Mac, and started using it for email. It keeps sending the same email repeatedly every ten minutes to the same recipients, for hours. Seems to happen with large files. The emails are in fact received, but keep getting sent. How do it stop it? A yellow warning ! icon message says Connection to server lost or failed. The small window at the bottom of the screen shows the rolling blue bar trying to send during all of this.