Office 365, Email, Outlook, Messages aren't sent and appear in outbox folder

Sent messages in Microsoft Outlook appear in the Outbox folder, instead of the normal sent mail folder. This is accompanied by a message saying that the messages were not sent.

Go to File -> Options, and then click on the Advanced tab. You should see a check box for "Send immediately when connected." Make sure this is checked. This is checked by default.

You will also want to check to make sure that Outlook is not in "Work Offline" mode by going to the Send/Receive Tab, in the Preferences Group and making sure that Work Offline is Not Active.