Results skewed by headers

[Stephen Davies]

I am seeing a number of missed spams due to header counts - particularly from, 
rcvd and url - being one.

That is, I see results such as:
X-Bogosity: Ham, tests=bogofilter, spamicity=0.535987, version=1.2.0

  "from:butlerbuilder.com"              1  0.000079  0.000000  0.009094 +
  "from:welterweight00"                 1  0.000079  0.000000  0.009094 +
  "url:190.159.9"                       1  0.000079  0.000000  0.009094 +
  "url:190.159.9.229"                   1  0.000079  0.000000  0.009094 +
  "easier"                           3151  0.058157  0.007897  0.119560 +

  "Acai"                            16862  0.002535  0.054990  0.955924 +
  "subj:Acai"                       56342  0.002535  0.183987  0.986407 +
  "to:sdc"                          22925  0.000158  0.074898  0.997888 +
  "rcvd:sdc.com.au"                 89855  0.000079  0.293588  0.999730 +
  "rcvd:for"                        93890  0.000079  0.306772  0.999742 +
  "rcvd:sdc"                         9465  0.000000  0.030926  0.999999 +
  "rcvd:forged"                     18247  0.000000  0.059620  1.000000 +
  "rcvd:may"                        18248  0.000000  0.059623  1.000000 +
  "head:X-UIDL"                     40005  0.000000  0.130712  1.000000 +
  "rcvd:with"                      120472  0.000000  0.393629  1.000000 +
  N_P_Q_S_s_x_md                       15  0.000000  0.071974  0.535987
                                           0.017800  0.520000  0.375000

The first five apparently outweigh the negative results.
As soon as I run this through bogofilter -Ns, it becomes recognised as spam.
This seems to confirm that the from and url headers are the most significant.

Why is this and what can I do to stop it happening in future?

Cheers and thanks.
Stephen Davies

-- 
=============================================================================
Stephen Davies Consulting P/L                             Voice: 08-8177 1595
Adelaide, South Australia.                                Fax  : 08-8177 0133
Computing & Network solutions.                            Mobile:040 304 0583
                                          VoIP:sip:1132210 at sip1.bbpglobal.com