Ours tended to get hoarded in the recipients' desks. For some reason most of us always seemed to get more internal post than we sent.
That'll be the one on the right in my screenshot in that post; the orange one on the left was the one with the not very sticky self adhesive bit.
We would be using these envelopes right up to the early 2000s even after e-mail came in.
...and all this reminds me of triple carbon paper requisition forms, to be written in with what you needed, sent to your line manager for signing and then sent on to the Purchasing department who would actually place the order.
On one occasion, I had a large batch to do in one go, something like 10 or 11 different requis. I had long since had a suspicion that the managers were just blindly signing them without really reading them, so I wrote an extra one which I slipped into the middle of the pile. This extra one was for "a grey pachyderm on four legs with two metres of flexible trunking attached."
The manager signed it...