Sent text goes to one recipient's email instead of their mobile number?

I used to send texts to this one recipient with no issues; last one was July 2020. Those all appeared to go thru as iMessages. Then is Aug 2021 I sent them a text and it went through as an email to their email account. For some reason that msg showed up as green/SMS msg on my iphone, instead of blue/imsg as previously. I have this same recipient on a group text with another person and all texts there go directly to their phone as a text and show up as blue/imsg when I send. Last msgs sent there were Dec. 2021.

When I attempt to text this person individually the text automatically defaults to their email address. It doesn't matter if I attempt to create the text in the Message app, or if I open the contact's information under the phone app, and click on Message. I'm on IOS 15.1. How do I resolve?