Outlook Express 6 Forwarding FPs
Messages forwarded automatically by a Message Rule in Microsoft Outlook Express 6 may be marked as junk mail because of a bug in Outlook Express.
If the action of a Message Rule is set to Forward messages, Outlook Express correctly changes the From: header of the forwarded message but does not correctly generate a new message ID, in violation of RFC 2822.
If the domain name in the message ID does not match the domain name in the From: header, the message may hit the SpamAssassin 'MID_ADDED_BY_RELAY' rules, and therefore may cause a false positive.
This is a bug in Outlook Express, and there is no known workaround. For further details:
Update: Mar 2007: the rule is no longer part of SA, removed some time before 3.2.0.