Keep them if they write a message on them. Chuck them otherwise. A new envelope costs so little you'd be pretty cheap to re-use.

Throw them away.

Personally, I might reuse if its clean enough otherwise I just use a new one each time, I like the fresh feeling from a new envelope.

I will strip the card from it and use that in other parcels and sometimes reuse stamps if they pass through the system unstamped, any stamped stamps are saved for a charity which raises money with used stamps.

Hi everyone I know this might seem strange but I wanted to know what you do with your return envelopes that you receive? I was trying to figure out if I should reuse mine and put on fresh postage since it already has by address on it. I could use another envelope but I was wondering what you all do. Thanks