SMTP service in MailEnable Standard, Professional, and Enterprise before ME-10014 (20060904) allows remote attackers to cause a denial of service via an SPF lookup for a domain with a large number of records, which triggers a null pointer exception.
https://exchange.xforce.ibmcloud.com/vulnerabilities/28910
http://www.vupen.com/english/advisories/2006/3669
http://www.securityfocus.com/bid/20091
http://www.mailenable.com/hotfix/MESMTPC.ZIP
http://www.mailenable.com/hotfix/