Reason for asking is that I have just received two emails supposedly from Egg but with this and if I knew the effect of increasing the spam filter sensitivity I might do it

I don't recall an explanation being posted, and last time I searched on IronPort's website and on  Google I drew a blank. My impression is that PN call an IronPort-supplied black-box function passing the current aggressiveness level as argument, and spam/notspam is returned.