This kind of bomb is actually send out from unknown address, great amount, huge capacity, full of chaos codes and nonsense mails. Most of the time send away same mail repeatedly.

这类炸弹实质上是发送地址不详、数量巨、容量庞大、充满乱码或没有意义的垃圾邮件，大多数时候通过大量重复发送信件来实现。

youdao

Dietz worked on the Washington sniper case as well as the "Unabomber" mail-bomb case.

CNN: L.A. manhunt reminiscent of D.C. sniper case

Suspicious parcels, reported bomb threats and seemingly odd pieces of mail prompted evacuations and emergency alerts from Washington to Arizona to Michigan.

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Army bomb experts were called to examine the package, which had been taken from the Royal Mail sorting office.

BBC: Neil Lennon bomb police probe fifth suspect package