I *think* the cell phone spammers use a type of "wardialer" for email addresses. Correct me if I'm wrong, but if your provider supports it, you can email your phone by sending an email to [yournumberhere]@[yourcellprovider].com
All spammers have to do is bulk mail out to every number they carry. With the massive proliferation of cell phone numbers, they probably hit 60% of the time.
I think its legal too, as legal as spam is (because it's a list of numbers, and not dialing each one of them).
I would call T-mobile and flip out, especially if you get charged for mobile email.