Hi Arashghanbar,
Thank you for the update on the issue.
As per the screen shot which you have provided it seems that the entry which is showing unknown app in Credential manager is associated with some website as it is showing Website address (URL).
There might be a chance that you have logged in on some website and used your email credentials and by mistake you have selected save this option.
I would request you to click on show option infront of password and check the password which might help you to find out the credential where you have used them and then you can move ahead to know the source of the unknown app.
hope the information provided is helpful. Do let us know if you have any more concern related to Windows.
Regards,