It's in there, in the first post 

 Not a full blown email client (it's just a notification program), but the issue doesn't reside within the/an email client (if he even uses one, because that would be a different program; different process running on his machine entirely). I strongly believe that it's this program he's using for the notification. The notifications are invoked by this notification program, so this would be the issue in my mind.
POP Peeper website I found: 
POP Peeper - Email Notification
Although i've never used this before, and I don't feel like downloading it because I use Outlook. I've also never even heard of the program, so I can't say anything about it.