While having a bit of a sort-out, I found a rejection slip from Oink comic from 1987 (it was kind of a Viz for kids).
It was probably the first rejection slip I ever got. I was 19 years old and, being a teenager, was depressed by it and never sent them anything else! But what's interesting to me now about it, after years of bog-standard "sorry, no thanks" rejection slips, is the fact that it's a proper letter and is incredibly positive. If I got a rejection letter like this now I think I'd be pretty happy about it!