How can you distinguish between SMS and MMS if the message is short and contains no media?

I sent a short message to someone, and I thought I was sending an SMS, but when I asked them if they have read my SMS, they corrected me, and said "yes, I read your MMS". How did they know it's an MMS? What's the difference in this case? I know MMS allows for longer text, and images/videos, but my message was short and contained no media, so how did they know it was MMS and not SMS? What gave it away? Note that the CRS was enabled. Is this what gave it away? We both use Samsung phones, although mine is newer.