I don't claim to be an expert, it's really just common sense. Bombs are placed in the open or in things that are wooden, glass, plastic or cloth for a reason. Plastic explosive, high grade explosives or just a lot of explosives (1200 pounds in Dar es Salaam embassy bombing, 2000 pounds in Lebanon, 3-5 thousand at Khobar towers) will destroy anything but they aren't easily concealed in something as small as a mailbox (especially if there are route sweeps or bomb sniffing dogs around). Trash cans could be a good concealment point but they also will prevent maximum effectiveness of a bomb, they are used in Iraq all the time. But the ones that always did the most damage are the ones that had no obstructions. In neither Iraq or Afghanistan were we ever all that worried about trash can bombs, it was the ones just laying on the side of the street or placed under a table in a market that were most devastating. They tried to put them in oil drums, just made a loud noise and a mess and an oil drum is open on one end.