An SMTP service installs by default as part of Windows 2000 server products. Exchange 2000, which can only be installed on Windows 2000, uses the native Windows 2000 SMTP service rather than providing its own. In addition, Windows 2000 and Windows XP workstation products provide an SMTP service that is not installed by default. All of these implementations contain a flaw that could enable denial of service attacks to be mounted against the service. The flaw involves how the service handles a particular type of SMTP command used to transfer the data that constitutes an incoming mail. By sending a malformed version of this command, an attacker could cause the SMTP service to fail. This would have the effect of disrupting mail services on the affected system, but would not cause the operating system itself to fail.