One of our customers has a weird system where only some of the email
accounts seem to work with our event based notification.

There are 26 email accounts on the POA.
First I configured the accounts with the folderItemAdd event to send me a
notify whenever the event occurs.
I wrote a test app that will send email to all accounts.
Also I have the listener app.
17 out of 26 accounts work properly, I get the notification and am able to
get the events.
9 of the accounts have the event configure. But no events go into the
account, also, of course I get no notify either.

Any ideas what might cause such behavior?