-f function ???

Hello, I am using a mailing list script which makes use of a function called -f, according to the developers of the script. The fact is that some functionality of the script is not working and the developers claim -f is turned off in my system. I've looked everywhere trying to find info about this, but only found a small, not very enlightening, reference in the php manual under the function "mail." Anyone knows where to find info about -f? Thanks Guillermo Scharffenorth