Mail Security for MS Exchange 2003 - invalid rule

I am investigating as to why certain emails are being blocked, not coming in to our our domain email accounts.

I get the following message appear under the Event Log section of SMSME v7:

RULE: invalid rule

The following actions were taken on it:

The message "subject of email" was marked for deletion for the following reason: a filtering rule was violated

If I turn of Content Filtering off the email then comes in without issue

Any help, advice greatly appreciated