Why am I receiving messages from a group text as emails?
Two possibilities don’t include operator error.

Your cellular carrier has SMS to email enabled for your service.
The sender of the SMS has SMS to email enabled.

I would start with your carrier. This is easy to happen since AppleID and iMessages can be your email or AppleID or cellular number and the receiving party could have your contact commingled.

https://www.att.com/support/article/wireless/KM1061254/

Once you’re sure it’s not a carrier setting, you have to see the person doing the sending to be sure they don’t send to your email, but since you’re seeing the ATT mms email, I suspect the first two cases.