Email receipts can sometimes take up to 15 minutes to be delivered. Additionally you need to make sure that all the recipients emails are properly entered into the system. Lastly as you noted some users email client filter these notification into their junk folder. You should check with each recipient to make sure they have unblock the adobe domain in their email client.
I appreciate your response but the issue is still not resolved. Sometimes emails are received. Other times they are not. Everything described above has been done.