After having email notifications turned on for our PPM environment, we are not receiving email notifications from the system. If we change the email address on a resource to a personal gmail account or other, then they do receive the notifications to that account.
Typically, if the email notifications are not being received by a company email address, but are being received by another outside email address, this has to do with something being blocked on the email system not allowing those notification emails to be received by the end users.
The problem is on the customer's email server side. The customer's email system is blocking emails coming in from the CA PPM "from" address. The best way to fix this is to engage your email administration team to help find the reason why those emails are being blocked.