Microsoft Exchange Server 5.5 does not properly handle a MIME header with a blank charset specified, which allows remote attackers to cause a denial of service via a charset="" command, aka the "Malformed MIME Header" vulnerability.
https://exchange.xforce.ibmcloud.com/vulnerabilities/5448
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2000/ms00-082