Answer
A malicious email message is one which have been deliberately crafted to cause problems on the server or on the client. This could be due to the message containing a virus, or it could be due to teh message being crafted in such a way as to take advantage of a weakness in the receiving mail client.
GMS provides a range of checks which may be run against all messages passing through the system to prevent this type of message from entering the server at all. If you do not want to ban the messages entirely it will add a warning to the incoming message.
These checks are provided in addition to the standard malicious content checks such as virus scanning and attachment blocking. They are known as Message Quality checks, and inspect the content of each message to ensure it is structurally sound, as well as looking for structures that are typically designed to take avantage of flaws in some of the more popular mail clients.
The checks include checking line lengths, checking for the presence of clsid’s in mime attachment references, cid’s to load html content, suspicious attachments and verifying the integratity of mime encoding.
Keywords:malicious content virus