In the midst of SF's late 60's "New Wave," writers like Andre Norton were continuing to hold fast to tried-and-true old-school space opera conventions. No experimentation for this lady! Postmarked the Stars is the kind of story where "radiation" can cause strange and horrific transformations in living things instead of just killing them, and characters, when shocked or surprised, say things like "By the Five Names of Stayfol!" instead of simply "Holy crap!" Even upon its release, it was a quaint throwback to a simpler and less sophisticated age of pulp SF. Today, like so much of Norton's work, it's not much more than a curiosity.
To the novel's credit, though, its setup is still a humdinger, even if it does quickly lapse into pulp. Dane Thorson, assistant cargo master of the free trading vessel Solar Queen, awakens from a stupor in a dingy room of a port town on the planet Xecho. Some miscreant has tried to kill him! But why? Stumbling and staggering back to his ship just before it is to take off for Trewsworld, Thorson and the rest of the Solar Queen's crew are stunned to discover a man disguised as Thorson in Thorson's cabin, dead of heart failure due to the stress of launch. (Why a guy with a heart condition thought he could stow away on a spacecraft isn't really dealt with by Norton.)
Okay, so why was this guy on board? A quick check of the cargo, mostly exotic alien livestock, reveals no theft, so perhaps he was attempting to smuggle something onboard. Sure enough, the crew discover a strange box emitting a peculiar "radiation," and yet which, apparently, it is safe to handle with only a spacesuit glove. At this point, the book's science gets so hilariously bad your BS alarms will be going off like an air raid siren. This "radiation" has a startling effect upon the ship's livestock, causing them to — get this — regress down the evolutionary path to an earlier form! (That's right, the crew themselves aren't affected.) This has the most profound effect upon the small brachs, who suddenly display a surprising intelligence. Were the brachs once more advanced than they are now? Evidently so.
Okay, Remedial Evolution 101: Evolution is a process that happens to populations over many generations, not individual organisms. I don't care what kind of magic "radiation" you want to concoct for your story, nothing can force an organism to "devolve" into something it never was. Put this in your novel and, unless they're utter creationist morons or something, you're requiring your readers not only to suspend their disbelief but hurl it screaming off a cliff.
The story goes on to follow a conventional formula involving run-ins with criminal gangs, and the Solar Queen's crew desperately trying to maintain their innocence in what looks like a far-reaching conspiracy. There's not much in the way of thrills and chills, though, due mostly to the dated qualities of Norton's writing and the scientific absurdities of her plot. Unintended chuckles pepper the narrative as well, forever reminding you of the story's anachronistic nature. The biggest howler takes place early in the book while the crew is still on Xecho, in a scene where an alien woman greets Thorson with the honorific "Gentle Homo."
There's still a lot of older SF out there in the used book stores that holds up nicely after all these years. But odds are those books don't have Andre Norton's byline on them, I'm sorry to say. Postmarked the Stars? The stars can have it.