The case of the disappearing email - MacOS High Sierra 10.13.3

I know others have had this problem but I haven't seen a viable fix. This is a recent phenomenon for me. An email will come in (in this case a request for password reset for an app) and immediately disappear. No time to click on it or read it, just a quick glance to see who it's from. The email can not be found anywhere - not in inbox, junk folder, trash.. nowhere to be found. Vanished! This is only happening with one sender as far as I know but he (the sender) has not encountered this issue with others. Any suggestions to find these "disappearing emails?" They have to be somewhere.