This issue can occur when large emails are sent using Outlook
Click on the “Send/Receive” tab, then toggle the “Work offline” option. Once this option is toggled, close outlook, then wait 1 minute. Open outlook again, and make sure that the “Work Offline” toggle is still enabled.
Delete the contents of your Outbox folder. Open the Trash folder, and if the email in question is there, then empty the trash folder.
Once you have confirmed that there are no copies of the email left in either the Outbox or the Trash folder, close outlook, wait 1 minute and open outlook again before disabling the “Work Offline” toggle in the “Send/Receive” tab.
Make sure to contact firstname.lastname@example.org with the subject line of the email in question to clear out any pending copies that may still be on the server waiting to be delivered.