I found these Office Max white #10 Regular security envelopes while cleaning out a house. There are 40 envelopes in a box and they measure 4 1/8" x 9 ½" with a printed lining for security.
These are made from a sturdy weight paper with a glue flap that you moisten and seems to stick well. Combined with the printed lining they make for a very safe secure envelope where the contents cannot be read from the outside, or the flap opened easily. I guess that's about all you can expect from a plain #10 envelope. Although I prefer the envelopes that have a peal and stick flap over these glued flaps.
I pay most of my bills online, so I don't use envelopes very often anymore. When I occasionally do need to have envelopes, I usually just buy envelopes at the dollar store. Comparing these Office Max envelopes to the dollar store brand, I honestly cannot see any difference in quality, and the dollar store envelopes also have the security lining, so I would not waste even a dollar more for the Office Max brand.
So in the long run, I would give them a 5 rating, but just not worth the extra money.