What do you see in your mail server log file? you will see your ‘test email’ as you said it was delivered OK, and then the other notifications that have tried to be sent should also be there.
but in your ‘new’ test form check what you have actually entered in the notification? have you used any ‘override’ functions. if you have you may well have entered in some invalid information and your mail server is refusing to deliver the email. Only use these if you are 100% sure on how your mail server is set up and configured as there can be many ‘security’ settings that could be applied.