MailPost 5.1.1sv, and possibly earlier versions, displays a different error message depending on whether the requested file exists or not, which allows remote attackers to gain sensitive information.
https://exchange.xforce.ibmcloud.com/vulnerabilities/17954
https://euvd.enisa.europa.eu/vulnerability/EUVD-2004-1100
http://www.securityfocus.com/bid/11599