What is the ruling on working for mail delivery companies that may transport packages containing both lawful and unlawful items?

It is permissible to work for postal and parcel delivery companies if the transported materials are permissible. As for forbidden materials such as intoxicants, tobacco, usurious documents, and the like, it is not permissible to transport them, register them, or assist in their transport, due to the Almighty's saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression). This ruling is conditional on knowing about the presence of forbidden items. If one is unaware, then there is no blame, unless the majority of transported items are forbidden; in that case, it is not permissible to transport anything coming from that source.