A Denial of Service vulnerability has been discovered in the Courier mail server. Due to a flawed status code check, failed DNS (domain name service) queries for SPF (sender policy framework) were not handled properly and could lead to memory corruption. A malicious DNS server could exploit this to crash the Courier server.
However, SPF is not enabled by default, so you are only vulnerable if you explicitly enabled it.
The Ubuntu 4.10 version of courier is not affected by this.