When I'm texting, sometimes the receiver's email shows up Whenever I start a text, my contacts list shows email addresses instead of phone numbers.  Not always but very often. Sometimes a contact will be that way, then next time it won't.
Those emails are from iMessage, as you can receive iMessages with both your email and phone number. If the messages are colored blue, it is iMessage.

https://deciphertools.com/blog/2014_06_20_stop_sending_imessage_from_email/