Mobile carriers monitor all SMS messages to some degree. To ensure your message reaches your client, you have to make sure your message won’t appear as a potential violation.
The monitoring system uses machine learning to determine the sending rates and the content. If the content identifies as spam or if the same number sends out excessive messages over a short period, the carrier views them as violations.
Specifically in the U.S., the top reason as to why messages are getting flagged, throttled or blocked is due to non-compliance with the recently updated 10-digit long codes (10dlc) regulations. Companies and their messages have to be registered with The Campaign Registry (TCR) who, in turn, updates the U.S. carriers of a long code’s reputation score. The U.S. carriers then use that information to determine throughput for those companies and their 10-digit long codes.