An email with heavy attachment stuck in outbox
A message contains a large attachment exceeds the limit of Mail service. This type of heavy message gets jammed in MS Outlook – outbox. The mail does not get delete easily and shows transmitting. Even you will close the outlook this mail will not get deleted.
How to rectify the issue of Heavy attachment in Outlook
Stop internet services cancel mail transaction. Once this is done move heavy attachment files from Outbox to Draft. Now you are free to delete or compress mail attachment.
Go to Send / Receive Tab and click work offline in MS outlook. This will stop all the transaction of mail sending and receiving. Now right-click or drag-drop mail and move to draft folder or delete.

Recap to resolve Stack heavy attachment mail
Click SEND/RECEIVE > Work Offline.
In the navigation pane, click Outbox.
Disconnect the Internet and stop the process from the bottom scale of outlook and try to move the mail to draft item.
Option 2
You Can’t Delete a Large File from the Outbox many times just see the screenshot. Double click

Greyed showing bar following screen will open just Cancel all the process.

Another option and screenshot shared as follows just go to the top ribbon and select Send / Receive and Press Cancel all button.
Conclusion:
Small issue but many times you may face this issue while you are sharing a heavy attachment mail to your friends. And get stuck even after that your important mail to your boss also not processing. What you will do? Hope this article will help you.
Share your comments and remarks.