MS05-021 is not categorized as virus, worm, Trojan or backdoor. It is a critical vulnerability in Exchange Server 2003 and 2000, which allows arbitrary code to be remotely executed in the vulnerable computer.
If exploited successfully, MS05-021 allows hackers to gain remote control of the affected computer with the same privileges as the logged on user. If this user had administrator rights, the hacker could take complete control of the system: create, modify or delete files, install programs, create new user accounts, etc.
In Exchange Server 2003, the hacker would have to authenticate as an account in an Exchange organization in order to exploit this vulnerability. In Exchange Server 2000, the attacker does not need to be authenticated, and could exploit this vulnerability by connecting to an SMTP port on the server and issuing a specially crafted command.
If you have a computer running Exchange Server 2003 or 2000, it is recommendable to download and apply the security patch for this vulnerability. Click here to access the web page for downloading the patch.