The common/lib/naming-factory.jar is missing MailSessionFactory and SendMailFactory, making the javax.mail.Session JNDI functionality unavailable.
Note: as the classes themselves haven't changed in a couple of months, a quick workaround is to copy the naming-factory.jar from tomcat 5.0.24 into the common/lib directory of 5.0.25.
When I built 5.0.25, these two classes weren't compiled and therefore not included in the naming-factory.jar. They weren't compiled because I didn't have javax.mail.Session in my libraries, making catalina/build.xml's ${compile.javamail} condition evaluate to false. It's too bad there's no way to grab JavaMail off iBiblio or something like that.
OK, I've done a build with these classes, tested it (works fine), and noted it to myself as part of the release management instructions. I've also fixed build.properties.default to add a reference to the activation.jar.
I tested on tomcat 4.1.30 and it does not work either. I put the naming-factory.jar from 4.1.24 and everything worked again !
Then it's not a tomcat 5 bug, then. I suggest you upgrade to tomcat 5. I'm changing this issue to be for tomcat 4.
This is fixed in 4.1.31