Hello Melanie. I too have encountered this kind of thing and have written about it here on IUOMA Portal. 

I don't like this kind of disrespect. At the same time, I have to admit that Mail Art has such a practice, it has already become a tradition and it can hardly be eradicated. Perhaps the only thing left to do is to dose communication with those colleagues who radically destroy the work they receive and return it in such a destroyed form (why?) to the sender. On the part of the colleague, such an act may not be destructive, but simply an offer of communication and a kind humour. 

LOL you guys-- Look what I received today!

and here's the back:

Just for the record--I sent The Celestial Scribe my original Galaxy Watercolor (which is now the background).

I must say--I LOVE LOVE LOVE this!

Here's another recent one: 

I sent this face as a sticker---

and I received back:

Which I also really love.

I must say, I have also been mail art conversing with Wm Paley--and recently I used some images from his art in collages with my own art for a deck of cards that I sent back to him. He sent a very complimentary note about them, and our conversation continues.

On the contrary, perhaps, I use the double-sided color copies that Guido Bandiolini sent me of some of his works to make envelopes, 6 or 7 of which I nested (I thought creatively--they were so cool), and sent back to him. I never heard anything back--and he may have been quite insulted--I don't know. I hope not!

Yesterday, I received some painted/stamped postcard backgrounds from Thomas Brown--who said "use these as backgrounds or whatever." And on my way to sleep last night I thought of a couple of ideas for them.

I guess I take it as a compliment--someone likes my art enough to collaborate or extend it. I don't take it as criticism or ruination or disrespect. I take it as play--creative response. My work inspired someone to do something... Of course, not all art "lands" perfectly or well, and I think we have t hold a space for these allowances in the context of the delightful greater whole. 

I found this enclosed in a stuffed envelope I received early on:

Finally, I found this quote in Mail Interviews I and printed it out to include in some of my envelopes because I think it's just good to remember--

As long as there are people-artists-living alone here and there, confronted by problems (professional, formal, human, or social. Mail Art is likely to have a role to play in helping to alleviate those problems. What we must not do is allow ourselves to take ourselves too seriously---tendentiousness is a natural health hazard for the mail artist. –Dick Higgins

I sent a postcard with one of my Gelli plate prints. The other day I received a return postcard from that person. She had created circles as a first layer then scanned or copied my Gelli plate print over the circles. She signed it with her name. I don't think so.