The Internet Mail Service in Exchange Server 5.5 and Exchange 2000 allows remote attackers to cause a denial of service (memory exhaustion) by directly connecting to the SMTP service and sending a certain extended verb request, possibly triggering a buffer overflow in Exchange 2000.
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2003/ms03-046
http://www.kb.cert.org/vuls/id/422156