I was having a problem sending emails from my Apache CXF web service. The emails just wouldn't get sent from within an exception handler on the web service.
CXF (2.0.4 in my case) provides it's own mail classes (geronimo-javamail_1.4_spec-1.0-M1.jar and geronimo-activation_1.1_spec-1.0-M1.jar). The solution was to remove references to these libraries from the Classpath, and instead use the Sun libraries (mail.jar and activation.jar).
This led to my second problem. Now my CXF emails would send, but they were displaying "(no subject)" for the subject. A bit of digging revealed that the Geronimo mail jars were still floating around in my deployed webapp library folder. Completely purging the web app and redeploying it fixed this.