Content filtering rules in Messaging Gateway that apply dictionaries of words against message headers may not operate as expected and fail to catch keywords in Subject lines and other message headers. This primarily occurs when rules against message headers are only written with dictionary based matches rather than keyword or regular expression matching.
The internal data structures used for message header testing are only created if there is a content filtering rule which uses regular expression or keyword matching against the message headers. This means that if only dictionary based rules are enabled for scanning message headers, those rules are not likely to fire.
A simple workaround is to create a rule to match a string against the Subject line but which takes no action other than the "Deliver message normally" when it fires. This will cause the header matching data structures to be created which will allow the dictionary based rules to operate as expected.
The presence of this rule will cause the header structures to be created and the dictionary base rules to operate as expected.