i apologize for all of the typos!
We definitely want you to be able to use messages while using our DIGITs program. I appreciate that you already shut off iMessage. However, for it to send through the iCloud email suggests there is still a connection. Please make sure iMessage is off for all devices using the same Apple ID.
One of the most effective ways to do this, which often results in it being deregistered a little faster, is going to the following website: Deregister and Turn Off iMessage - Apple Support
Apple indicates it can take up to 24 hours for them to completely deregister you from iMessage so there is definitely a lag time from when you turn off the service before all of your contacts who have iPhones are updated.
This should allow your message to send and receive as normal SMS. Please let us know if you have further issues after 24 hours from taking that step.