Text messages sent to wireless phones via email (SMTP) are covered by the 
Can-Spam act of 2003. This act prohibits  sending unwanted commercial email 
messages to wireless devices without prior permission. The second link below offers 
a really great explanation, including a good description of what constitutes a 
commercial message: 

http://www.fcc.gov/guides/spam-unwanted-text-messages-and-email
http://business.ftc.gov/documents/bus61-can-spam-act-compliance-guide-business/