It appears that Simmons, Jay via NANOG <jsimmons@semmes-murphey.com> said:
-=-=-=-=-=- This may be the issue
Sorry, but no.
Here are some details on this Government protocol implemented by all Telecom Carriers.
Why it is being done? To support FCC mandate for STIR/SHAKEN, an industry set of rules designed to authenticate and validate CallerID information associated with phone calls using digital signatures.
SHAKEN/STIR ...
STIR/SHAKEN only affects voice calls. It has nothing to do with SMS. As several other people have said, if you're sending safety critical SMS messages, use a real SMS service, not a carrier's courtesy low volume e-mail gateway. SMS services are not free, but they are not expensive, typically about 1/2 cent per message. R's, John