症状

An error has occurred Configuring a Server for Outgoing Mail:

java.lang.ClassCastException: gnu.mail.handler.TextPlain
at javax.activation.MailcapCommandMap.createDataContentHandler(MailcapCommandMap.java:496)

原因

The gnu version of JavaMail and the Sun version of JavaMail are both in the class path.

ソリューション

Install the javamail package and remove the gnumailpackage.