An anti-virus system provider distributes an e-mail identifying content filtering rule seeking to identify e-mail messages suspected of containing an item of malware from a central source (20) to users (2). This distribution may be by an e-mail message itself which is appropriately signed and encrypted. At the user system (2), the received e-mail identifying content filtering rule is extracted from the e-mail message and added to the content filtering rules (18) being applied within that user system. In this way, malware which is distributed by e-mail may be identified by characteristics of its carrier e-mail rather than characteristics of the malware itself which not yet have been properly analyzed or the mechanisms for detecting such characteristics of the malware itself not yet put in place.