java.lang.NoClassDefFoundError: javax/activation/DataSource

[Alessandro Rossi]

Hi,
I have a struts application under tomcat.

From a class I have to send an email. When I execute the

Message msg = new MimeMessage(session);

it gives me the error in object.
But I added the file activation.jar to the libraries...
why I have this error?
If I add a jar to the libraries, this is not added to the classPath?

Thank you
Alessandro Rossi