I was running Outlook as my main mail app, and while doing that the notification alerts were showing with just the generic 'app' icon.
For various reasons, I switched back to Mail.app, but everytime I get a new email I get not only the (correct) Mail notification alert, but also the faulty Outlook alert. I use an Exchange account with push.
I have uninstalled Outlook, and rebooted my MacBook Pro.
Best Answer
Check your Internet Accounts in System Preferences in your Mac. Mac sure that it is deleted.