We needed to add a smtp to sms functionality at our sites, we use the RUT956.. Using python, we created a py script that runs via the custom scripts interface.. It runs as a service that monitors port 2525. It accepts emails directed to 1234567890@sms.local and routes those out as a sms. It also relays all other emails to smtp-relay.gmail.com with no auth since our wan ip is whitelisted for sending emails. Seems to work well.. Here is the script for your info.. Feel free to use or share..
Enjoy.