Today is the 11th of Tamuz, 5770
2000 years ago the rabbis of the Talmud forbade buying cow milk from a non-Jewish milkman. Why? Because the non Jewish milkman would have other animals than cows in his farm. At the time of collecting the milk he would normally mix it with camel milk, donkey milk, etc. which obviously aren't Kosher! The rabbis then forbade buying 'milk' from a gentile milkman unless the milking process was supervised by a Jew, making sure it is pure cow milk.
But, what happens today when the circumstances have changed? Should we still suspect that in today's food industry companies would mix cow milk with non Kosher milk?
There are three opinions on this issue and as we have explained, given two or more opinions everyone should follow his community Minhaguim.
Opinion # 1: the Rabbis made a decree and 'formal rabbinical decrees' must be derogated to run out. They don't expire by themselves, even when the circumstances have changed. Accordingly, milk has to be supervised.
Opinion # 2: The USDA inspection service serves the same role as the original 'Jewish supervisor', therefore any normal cow milk is permitted (R Moshe Feinstein z"l)
Opinion # 3: The Rabbis did not issue a 'formal decree' but a 'warning' (Rambam). Therefore, when circumstances change, the warning is not in effect anymore. The original warning assumed that the gentile milkman would have an extra profit from mixing milk (Chut haMeshulash). Today, non kosher milk would cost more than cow milk. No restrictions are necessary. Normal cow or goat milk is allowed.