I've recently received envelopes back that I sent a month ago, with the USPS stamps and stickers stating "no such number", " illegible", "unable to forward", "return to sender", and "postage due $3.65". And that was just one envelope. Bullspit!
From now on, on your request envelope that you send to me, put my address as the mailing address AND the return address.
On the SASE, put your address as the mailing address AND the return address. This way, if they try this crap some more, when they return it to the sender, it should still come back to you.
I've messaged the members whose envelopes came back to confirm addresses, then I will re-send the seeds.
Bless their little, letter carrying hearts.
Edit: I read through responses to this and I realized that it sounds like I'm suggesting something hinky myself. Let me be clear that you still need to put the proper postage on each envelope. Still do the return address trick thingy, but affix the stamps.
Sorry for any confusion.