This also happened to me! I sent a Christmas and New Years message to multiple recipients (only a simple text message). I checked my vodafone app today to see that I had $18.43 in extra charges. As I have unlimited texting and calling on my plan and 2.5gb of data I was surprised to see this much. I rang Vodafone and they said I had sent pxts/mms messages on 25 dec and 1 jan. I knew I hadn't sent any pxts and told them so and they said well you must have to which I hung up. Don't like being called a liar. Anyhow, I looked at the Vodafone app to see $18.43 in MMS charges. I looked at my message history at the time these charges were made and they were Christmas and New Years messages sent to multiple recipients, but they were only text messages (SMS) so I rang Vodafone back informing them of this only to be told that some phones convert messages to multiple recipients into mms messages as was the case with my iPhone 4S. There was no warning from my phone that this was happening, and it doesn't appear to be common knowledge as I've asked many people with iPhones who didn't know this happened either. Luckily, the lady at Vodafone cleared the charges as she realized that it was an innocent mistake and she could tell that I was unaware that this happened.
Is there a way I can stop this or do I need to send 40 individual messages at Easter this year?