Hi Sabahat!
The best way to eliminate this problem/exception is to remove the xerces.jar from tomcat/common/lib if you had copied it by mistake. This problem occurs if you have multiple parsers. Apart from the tomcat libs, you just need to have activation.jar and mail.jar in your tomcat/common/lib.
I wouldn't do that. Leave the xercesImpl.jar there alone.... or, minimally, upgrade to the latest release of Xerces and make sure to replace xmlParserAPIs.jar as well with the one that comes with the latest release (or with xml-apis.jar which is the preferred jar as the xmlParserAPIs.jar name is deprecated). Remove the XercesImpl.jar from Axis and move jaxrpc.jar and saaj.jar to CATALINA_HOME/common/lib. Of course, also add activation.jar and mail.jar as Sai has said. I would also upgrade log4j.jar to the latest 1.2.8, while you are at it. And, why not upgrade to a more recent Tomcat version than 4.0.4. That's some old stuff. 5.0.16 is the latest and very stable.
Anyway, things should work.
Jake
Hope this works Sai
