The remote host is vulnerable to a Denial of Service (DoS) attack.
The remote host is running Courier Mail Server, an open source mail server for Linux and Unix. The installed version of Courier is prone to a remote denial of service vulnerability associated with Sender Policy Framework (SPF) data lookups. To exploit this flaw, an attacker would need to control a DNS server and return malicious SPF records in response to queries from the affected application.
Upgrade to version 0.50.1 or higher.