The jar  file is actually a ziped version of
files.Actually a jar file  can be unzipped using the
winzip utility too!!!!

Make sure that your jar file internally contains a lib
directory. In the lib directory place the
activation.jar and also the mail  jar files.

The expanded structure of the jar file should be

somename.jar
|
|
|-----META-INF
|
|-----lib
|
|
|---activation.jar
|
|
|---mail.jar
|
|
|---pop3.jar (etc)