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!

 

It means that somewhere in your inbox there is an unsent message try deleting the one you find that has the same icon and see if it fixes your problem