Outlook for Mac: Emails don't get send but not in outbox

Hi

Working with Outlook 15.8 (150303) and I have a strange issue. When I send an e-mail it looks as if it has been send, but in reality it isn't. When I then switch into my Citrix session on which we run Outlook 2013 for Windows, I can then see the message in my "Sent Items" as a draft. When I then press Send, the mail is send. See screenshot.