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:



https://www.fcc.gov/consumers/guides/stop-unwanted-robocalls-and-texts

https://www.ftc.gov/business-guidance/resources/can-spam-act-compliance-guide-business