Which bin for envelopes?
Usual answer in the UK
Recycling Mixed recycling bin
Paper envelopes, including ones with plastic windows, go in paper recycling.
Common mistakes
Padded and bubble-lined envelopes go in general waste.
Does every council do this?
Most councils follow the answer above, but bins and sorting rules are set locally, and a few councils still do things differently. If your bin has a sticker, or your council sent a leaflet, follow that. Enter your postcode to go straight to your council's guidance.