Samsung Brightside

I need some clarification on the meaning of a couple text messaging icons. What is the difference between the unopened backside of an envelope with a red check mark in the bottom left corner vs. an unopened backside of an envelope with a red arrow pointing up in the upper right hand corner? I am going nuts trying to figure out what they mean in regards to the text messages I've sent to people. If anyone can help, I'd appreciate it.