Can anyone help me solve this Iphone texting mystery?

I was in Mexico for a week and only texted some people when I had wi-fi in the hotel. One of my friends didn’t respond to any texts; others did. Since I got home, I’ve texted this friend with no response and last night got a message from him saying, “Who is this?”

Apparently my texts to him are now coming in as e-mails from an old but still active e-mail account of mine that he didn’t recognize. Can anyone figure out why that might be?