I looked briefly at the activation/javamail dependencies and how these supposedly can completelyreplaced with geronimo specs and implementations.
I think the implementation classes within activation-1.1.jar won't be needed/used then (or at all even).
Documentation about replacing javamail with geronimo is very limited, but the following blog post seems helpful, especially in the context of cxf: http://sdomsta.blogspot.com/2009/03/java-mail-smtp-transport-provider.html
If I understand it correctly, the activation and javamail jars can be excluded if we just add an dependency on the Geronimo JavaMail Bundle pom and explicitly exclude activation/javamail from whatever dependency pulls them in:
That one will pull in all the needed dependencies:
Testing if it really works without the activation/javamail jars should be easy by configuring an smtp server and trying to send an email through the self registration portlet.