Many people use telecommunications provided SMTP to SMS/MMS gateways to send out sensitive data. This paper looks into encryption (or lack of) covered by these types of public access SMTP to SMS/MMS gateways and services.
You can download it from the following link: https://packetstormsecurity.com/files/download/111841/telco-crypto.pdf
Source: https://packetstormsecurity.com/files/111841/Telco-SMTP-To-SMS-MMS-Crypto.html