We all receive junk mail and regular bills from such as Orange, credit card companies etc, the potential list is endless.
Why not open the envelope before you bin it, turn it inside out and view the security overprint (intended to make the paper more opaque)
Such patterned printing can be used as backing papers on such as cards and scrap booking etc
If you really like the patterns, Try scanning them into your PC save and print some more. You could even change the colouring in most graphics programs.