Funny text message symbal

Hi,
I have received a message in to my inbox (benq siemens ef61)and I have never seen one like it before.
instead of showing as a shut envelope that you then click on to read-it came through as a opened envelope with a small box/square shape in the bottom right hand corner of the envelope (poss representing a peice of paper?).
It let me read it as normal but it is bugging me as to what it meant.
any ideas? help?
many thanks....