Hello

Sorry for the Inconvenience, 

Make sure outbox is empty and try sending emails to different user after performing below steps.

Click on MSN Explorer icon ( Butterfly icon)
Select the email account from the welcome screen (do not Sign in)
Press Ctrl+Shift+F9 at the same time
You will get the confirmation Pop up stating we are about to remove all MSN Data
Please Click on OK

Re-launch MSN it will download all the emails.

If Issue persists please follow the below link to contact the MSN Technical support team.
https://support.microsoft.com/en-us/contactus/msn/explorer-tech-services/

Thanks
MSN Support Team