Cannot open this item. Outlook has already begun transmitting this message.

The infamous “Cannot open this item. Outlook has already begun transmitting this message.” error is very common to frequent users of Outlook. During the Send/Receive process emails often get stuck in your Outbox and won’t get sent no matter how many times you run the Send/Receive process. Opening or deleting the message is not possible either.

There are a multitude of solutions to this online, but one that’s often overlooked and successful in many cases is to use the Mailbox Cleanup tool. To do that, first go offline by clicking File -> Work Offline in Microsoft Outlook. Then go to Tools -> Mailbox Cleanup… and perform a search. You can specify a search criteria to help you locate the message, for instance by attachment size or by period of time, and then you can sort by folder in order to see the items in Outbox. You should now be able to open the email and delete it.

In some situations you’ll be able to pull this off even without using Mailbox Cleanup, just by going offline using File -> Work Offline.