Its legal in the US so long as you follow the CAN-SPAM rules:
Federal rules require the following for commercial email sent to your mobile phone:
- Identification – The email must be clearly identified as a solicitation or advertisement for products or services;
- Opt-Out – The email must provide easily-accessible, legitimate, and free ways for you to reject future messages from that sender;
- Return Address – The email must contain legitimate return email addresses, as well as the sender’s postal address.
http://www.fcc.gov/guides/spam-unwanted-text-messages-and-em...
