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.