Buffer overflow in the base64 decoder in MERCUR Mailserver 4.2 before SP3a allows remote attackers to cause a denial of service and possibly execute arbitrary code via a long (1) AUTH command to the POP3 server or (2) AUTHENTICATE command to the IMAP server.
https://exchange.xforce.ibmcloud.com/vulnerabilities/13468
http://www.atrium-software.com/mail%20server/pub/mcr42sp3a.html