A computer system can be configured to identify when it has been infected with or otherwise compromised by malware, such as viruses, worms, etc. In one implementation, a computer system receives and installs one or more decoy contacts in a contact store and further installs one or more malware reporting modules that effectively filter outgoing messages. For example, a malware reporting module can redirect messages with a decoy contact address to an alternate inbox associated with the decoy contact. The same malware reporting module, or another module in the system, can also generate one or more reports indicating the presence of malware, either due to detection of the decoy contact address, or due to identifying messages in the decoy contact inbox. The host computer system that sent the message to the decoy contact can then be flagged as infected with malware.