Close Outlook (that will most likely be stuck into Outlook is transmitting the message mode…).

Start Outlook and activate the Offline mode: you can do that from the Send/Receive tab, and/or following these MS support guidelines.

Luckily enough, as pointed out by this great MS support post, the problem can be easily fixed by strategically using the Outlook Offline mode feature: this is what we can do to prevent Outlook to automatically try to send/move the e-mail message, allowing us to delete it. When this happens, even restarting MS Outlook or even rebooting the machine won’t fix the issue, because the software will most likely go in auto-send (and auto-move in Sent folder) mode right after its initialization… thus blocking the offending message there. The fix, needless to say, is to find a way to delete the offending message from the Outbox folder: problem is, MS Outlook won’t usually allow to delete message that are in the process of being sent… or moved to the Sent folder… Which takes us back to square one. The issue is often due to the fact that Outlook seems to be unable to move the e-mail message from the Outbox folder to the Sent folder – which is something that happens after the e-mail message has been actually sent: this makes it even worse, because it means that there’s an high chance that the receiver will be flooded by a number of identical e-mail messages until we figure out how to get rid of the problem. Sometimes, when you send an e-mail message using MS Outlook, it might happen that it gets stuck in your Outbox folder: such a scenario happens to be fairly common when the e-mail message has some rather big attachments (2mb or more).

Outlook hangs on send receive