Separate 'New Email Notification' box

Good afternoon.

A colleague of mine has started receiving a separate New Email Notification window/box whenever he receives a new email. We have tried to disable every setting related to notifications when receiving a new email, but it kept happening nonetheless. The window in the screenshot attached shows up and there is a 'beep' noise (despite the sound setting also being turned off).

Any suggestions on how to remove this?

Kind regards,
Antoine