They do not know whether the earnings of those who send food to the mosque are halaal
Question

                During ramzan a lot of muslim brothers send iftar items to the mosque and the people do the iftar with this items.what will happen to our fast if we do the iftar with the items send by a person whose earnings are haram(like bank employee,etc). we cannot know who send what in the masjid. Please clarify this.
Praise be to Allah.It is not essential, indeed it is not wise and it serves no interest, to ask about 
    the source of income of those brothers who send food, because that involves too much unnecessary effort. The basic principle concerning the 
    wealth, food and drink of the Muslims is that it is permissible unless it becomes apparent to you that it is otherwise. 
If a Muslim knows that this food was bought with haraam earnings, then he should 
    avoid it.  
We have already answered several similar questions. Please see nos.
    5559, 4820,
    37711 
And Allaah knows best.