Okay, then what is the significance of the number zero on the :0 line?
Nowadays, none really. In old versions of Procmail, you had to tell it how many condition lines your recipe contained (yes, “bletch”). The syntax was later extended and the number zero special-cased to mean, all the following lines which begin with asterisks are condition lines. (The asterisk at the beginning of each condition is obviously also part of this extended syntax.
