Not receiving verification codes

My Comcast email will not receive verification codes from a certain app - it is very frustrating because I cannot access important account info. There is no backup option either. I checked junk/spam folders, trash, tried logging in on different web browsers and the emails with codes were never received. I even signed up for a new account on the app using a Gmail to see if there was a problem with the app or my Comcast email but the codes sent in seconds for the Gmail account. I really need help troubleshooting this to access my account.