Texting from Galaxy S4

I have a question, I hope someone here can answer. My fiancee just got a Galaxy S4 from Sprint, I have a Captivate Glide, AT&T. When she sends me a text message, I have to download it, like an MMS msg. I find that quite weird! If it helps, the contact number the text comes in on, is Sprint's text messaging e-mail address. If I'm not mistaken, that format is used if you want to send a text from an e-mail program. Is there a reason why her texts would be coming through using this address? I'm stumped! She says, my texts to her come through fine. Can anyone help? Thanks!!