Email to Text

So I have seen the posts about the service not being constructed to handle large amounts of SMS messaging especially when it is emailed to text.  My question is what is considered a large amount of messages?  I created the system that my company uses to send out SOS messages for staffing shortages.  My system perhaps sends 15 individual messages to Verizon customers and others that are piggybacking onto your network.  The messages are being bounced back to our email and I am wondering if this is because of the Campaign Registry.  Thank you for any information.