I've read through all of the previous notification issues, and I think mine is unique.
I have a form that was sending notifications but quit, at least for the admin notification I set up. It is configured to send to my personal address and another email in the BCC. I have the person's name and email in the From fields to make it easy to add them (if necessary) to my address book. Nothing in the Reply To field.
This form was sending notifications through the old default Admin notification that was in place before the recent updates to GF, but I had replaced it with a new one. The new one worked for a bit but quit. The test notification I used was a Gmail address, so there cannot be an issue with the server seeing it as spam/phishing.
I installed the Configure SMTP plugin but I don't like that everything comes through on the Gmail address I configured it with instead of the email from the form. I really want to uninstall the SMTP plugin, and need GF to start working correctly.