Check your contacts for the number. If it’s someone you know, they may have your contact information stored wrong in their phone. One of my users (I work in I.T. support) frequently texts me to my email, and I get little text attachments saying
Again are you using Apple's email account? Like icloud or something else like yahoo or gmail? IF that latter those are Apple email account just because you're using the mail app doesn't mean it as an iPhone or Apple issue.