to send = بَعَث
بَعَث is to send. When the object is a person, the verb is transitive, so we would say بَعَثَ الْمَلِكُ رَسُولاً إِلَيْهِم the king sent a messenger to them. However, where the object is inanimate, the verb is used intransitively, with the prepositional complement ب , e.g. بَعَثَ الْمَلِكُ بِرِسالَةٍ إِلَيْهِم the king sent a letter to them. The reason for this is that the verb بَعَث is connected with the idea of rousing someone into action, hence it can have the meaning to raise the dead, e.g. إِنَّ الله يَبْعَثُ الْمَوْتَى يَوْمَ الْقِيَامَة God will raise the dead on the Day of Judgement, or to raise someone from sleep, e.g. اِبْعَثْهُ مِنْ مَنامِهِ wake him up from his sleep. Hence when it is used to mean to send, it is understood that the object being sent is in some sense being put into action. Given that this is not the intended meaning when the thing being sent is an inanimate object, we therefore use the preposition ب , almost as if we were saying, in the aforementioned example, the king sent (a messenger) with a letter to them.