Zoroatrianism is a very interesting religion. its not a paganic religion, but a dualist one. there are two gods by this religion.
so when Islam get to persia, they didnt treat them like paganism.
as i understood, it was mainly a State religion, not of the sinmple man.
It also saw fire as a holy element, like jewdisem and christianity and Islam see water.
{anyone who played rome total war - barbarian invasion whould rememmber this religion :)}