What controles this 'spam(no null-header or sender address)'

[Carl Boberg]

Hi,

A while ago we started getting mail marked as spam that where from mailinglists automatic senders and others with empty return-path : Return-Path: <g> or similar

They get flaged as spam with no spamscore and just this statement in the Spamreport (Mailwatch):
spam(no null-header or sender address)

How can i stop this behaviour?

Cheers
-------------------------------- 
Carl Boberg 
System & Network Administrator 
Swedish Museum of Naturalhistory 
Frescativägen 40 
104 05 Stockholm 
Sweden 
Tel nr: 08-5195 5116 
Mobile: 0701-82 4055 
E-mail: carl.boberg at nrm.se 
--------------------------------