This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
Add the -verbose:class option to JAVA_OPTS within run.bat (or run.comf on Linux). This option prints out each class loaded and the JAR file it came from. This should help you located the addition JAR file containing javax.mail.Session.