There were these guys named named Mike Cook and Tim Turner. They published something called the 24-7 Artzine out of Providence, Rhode Island in the mid 1990s. It was a modest xeroxed rag, dropped off in the doorways of comix shops and record stores and filled with lots of weird poetry and art reviews. I used to send in little drawings like this one.
Brooding paintress Lora Shelley is easily the most prolific of all correspondents I've had the pleasure of knowing. Her letters are full of doodles, sketches, and unabashed introspective personal nuance. This envelope here is in my coveted Envelope Hall of Fame... I'm saving up for a helium-filled glass case to house it in! You can buy prints from Lora here.
Oh, and here's an envelope drawing for Lora from several years ago.