As ridiculous as this sounds, the only real fix I have found to work is to delete the offending email account from your iPhone and the add it back. That seems to clear it.
You need your username and password to add your account, so be sure you have it before you delete it!
When you are adding the account, there will be a list of accounts, including Gmail, AOL and Outlook.
Even if your email account is an @live.com email address, instead of @outlook.com, choose Outlook.com.
After 30 minutes the message had not shown up again.
If I am correct, the account settings are changed from an imap to pop3. It doesn't matter to me HOW I get my emails, as long as I get them securely!
Good luck!