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.
And Allaah knows best.