1.) Go into C:\Users\your username\AppData\Local\Microsoft\Windows Mail\Local Folders (note that the directory is hidden by default) and find/delete the message directly be looking for the matching date/time stamp.
2.) Go into the webmail account and delete it directly from there as opposed to using the e-mail client.
Hope this helps,
I get message Unknown error has occurred. It happens with only 1 email. I read somewhere to delete the WindowsMail.MSMessageStore but that didn't solve the problem.