Avoid redundancy. If you have your jars under /libs in your app build.gradle by default you will have
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
...
}
that is enough to add all the jars you have under /libs
this is not necessary
// compile files('libs/activation.jar')
// compile files('libs/additional.jar')
// compile files('libs/mail.jar')