Don't see much to complain about with your local.cf, other than the two
obvious 2.6x lines that weren't updated. As others have mentioned, taking
the autolearn ham threshold down to 0 or -0.1 would be a real good idea.
I'm not sure about those last two add_header lines. I'm assuming that is
something local that may interface to the rest of your mail system, perhaps
for spam filtering.
You also want to look at which plugins you have enabled. Those are in a
separate file (or two files for 3.1) the names of which escape me at the
moment. You should have surbl enabled for best effect.
Loren
