Harry thought he could figure out who the Death Eaters were by checking their arms for the Dark Mark. Turns out it's not that simple.

Like I said, Voldemort is one of the only two or three competent wizards shown.

Does either of you even have any reason to believe that there isn't a magical precaution against sending dangerous objects through the owl mail system?

It's more likely that there isn't than that there is, and it's something to be (quickly and easily) tested. This particular example aside, owling bombs isn't intended to be taken literally; it's a stand-in for a simple attack that wizards don't expect. Any particular such attack may not work, but I strongly expect that if Harry dedicated a whole day to thinking up and actually trying such "low-tech" attacks, vetting them with Dumbledore against known defenses to save time, then he could come up with enough attacks to take down at least half the Dark Lords in recent history.