Welcome to Android Central! This is a common problem on AT&T -- see this thread:
https://forums.att.com/conversations...6e8c5d2#M40808. Not sure if the proposed solution there actually works. Other carriers (like T-Mobile, which I use) have an option within the user account website to disable the email address associated with the phone number (and which then converts the email to a text). You could try digging in your AT&T account website, or talk to customer service about it, but from the many forum threads that come up with a search about this issue, it seems like AT&T has never come up with a solution.