what does the icon mean...

I am trying to figure out what the icon to the right of my text messaging screen means. I see the list of people who I've texted and on some, to the right, it will say the time the text was sent/received. However, when I send a text to certain people, there's an icon below the time that looks like a white envelope w/a red "x" in the corner of it. I can't figure out what that means. Anyone know? I asked one of my text recipients if they're getting my texts and they were. So, I'm not sure what that icon means. Can someone please help me?! Thanks!