I have worked with TWO customers today with the same issue, and I saw it firsthand. Both customers are on Yosemite. One was using Outlook for MAC 2011 and one was using MACMail.
Emails that are being sent from the Drafts folder re-appear in the Drafts folder after being sent. The emails are sent and do appear in the Sent Items folder. However, they also re-appear in the Drafts folder as if they haven't been sent. I was connected to both customers' MAC's via Remote Support, and I even tried to delete the emails, but the emails re-appeared. I closed MACMail and re-opened it and still the emails are in the Drafts folder. My other client has also closed Outlook 2011 and re-opened it, only to see the emails still in the Drafts folder. This is VERY confusing to both clients, and I am scratching my head on this as well. Any thou