Outlook Web Access (OWA) in Microsoft Exchange 5.5, SP4 and earlier, allows remote attackers to identify valid user email addresses by directly accessing a back-end function that processes the global address list (GAL).
https://exchange.xforce.ibmcloud.com/vulnerabilities/7089
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2001/ms01-047
http://www.securityfocus.com/bid/3301
http://support.microsoft.com/support/kb/articles/Q307/1/95.ASP