Office 365, Email, Outlook, Messages aren't sent and appear in outbox folder
Send immediately when connected
In Outlook's main window, click the File tab in the top ribbon.
In the lefthand pane of the File page that appears (with the heading Account Information), click Options close to the bottom.
In the Outlook Options window that appears, select Advanced on the lefthand pane.
Under the "Send and receive" heading, make sure the "Send immediately when connected" box is checked.
In Outlook's main window, click the Send / Receive tab in the top ribbon.
Make sure that the "Work Offline" button in the Preferences group is not highlighted, and that "Connected to: Microsoft Exchange" appears in the status bar at the bottom of the main Outlook window.