Because of different interpretations of standards in mail clients, IDS/IPS and antivirus products, it is possible to pass malware undetected to the end user. This is especially funny and dangerous if different interpretations happen inside a single product, like in Yahoo! Web Mail.
more here.........http://noxxi.de/research/content-transfer-encoding.html
more here.........http://noxxi.de/research/content-transfer-encoding.html