The forum to email gateway is producing headers > 998 chars in violation of RFC 2822.  In specific, the References header is too long, and was detected by my mail server.  In one particular thread, references is over 1200 chars:

References: <ricks.cafe.23vxen1141039300@no-mx.forums.slimdevices.com><etychon.23w9ob1141055218@no-mx.forums.slimdevices.com><ricks.cafe.23yjun1141161740@no-mx.forums.slimdevices.com><Millwood.2422gz1141325850@no-mx.forums.slimdevices.com><Siduhe.2422gz1141325851@no-mx.forums.slimdevices.com><ricks.cafe.2423eb1141327052@no-mx.forums.slimdevices.com><fairyliquidizer.24q47n1142447102@no-mx.forums.slimdevices.com><dva184$5k5$1@sea.gmane.org><fairyliquidizer.24rhab1142510701@no-mx.forums.slimdevices.com><fairyliquidizer.24t80n1142592002@no-mx.forums.slimdevices.com><Triode.24ttbb1142619602@no-mx.forums.slimdevices.com><govardha.24xajn1142781905@no-mx.forums.slimdevices.com><fairyliquidizer.24zjc01142886605@no-mx.forums.slimdevices.com><Triode.24zjsn1142887201@no-mx.forums.slimdevices.com><fairyliquidizer.24zrnz1142897403@no-mx.forums.slimdevices.com><govardha.25auqn1143414602@no-mx.forums.slimdevices.com><fairyliquidizer.25chb01143490503@no-mx.forums.slimdevices.com><Apteryx.25dgyc1143536701@no-mx.forums.slimdevices.com><Triode.25e7cb1143570901@no-mx.forums.slimdevices.com><20060328103956.cd0a4pmkkkokcs44@www.deane-freeman.com><Triode.25e8qb1143572702@no-mx.forums.slimdevices.com><MrC.25e96z1143573301@no-mx.forums.slimdevices.com>
It seems References is being used as the CC list.

This can trigger anti-spam behavior.