I guess (based on the skeptical reactions) your experience is atypical now-a-days, but based on your comment, I did find a 2014 wired article saying that while previously unheard of, iMessage now [then] accounted for 30% of spam, from email addresses like you described. I had no idea. Somehow I avoided that. I didn’t find anything more recent, though, so maybe it’s less of a problem now?

https://www.wired.com/2014/08/apples-imessage-is-being-taken...

I do know first hand that Apple will turn off (starting at several days) an Apple user’s ability to use iMessage if they get even several reports, though.