If a bunch of people in the same area sent improperly packaged magnets or some magnets and some ferrous objects, things could get amusing.
Remember those buckyball magnetized balls?
You could also cover the outside in random barcodes, bonus points if you know what sequences you need to trip internal routing systems. Alternatively, covering them with random addresses could be like a captcha for the mail sorters, getting letters stuck in an infinite loop until humans intervene.
It would be interesting to see if the mail systems have been sufficiently tested against this style of attack.
I would actually argue that is the best strategy - not dangerous objects, just envelope/package layouts that prevent proper automated routing and end up circulating in the system until someone catches on and has to pull them.
Under normal situations, this would be rare and most people design their letters to minimize the odds of incorrect routing, but when you inverse the goal…. Is the system secure against it?
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