Android-从ProGuard中排除库(jar)类

我试图混淆我的项目,但我得到了以下例外:

Unexpected error while evaluating instruction:
Class       = [com/sun/mail/pop3/Protocol]
Method      = [quit()Z]
Instruction = [24] aload_3 v3
Exception   = [java.lang.NullPointerException] (null)
Unexpected error while performing partial evaluation:
Class       = [com/sun/mail/pop3/Protocol]
Method      = [quit()Z]
Exception   = [java.lang.NullPointerException] (null)

我试过了: -keep class com.sun.mail.pop3.Protocol{ *; } 但这无济于事。如何将此类从ProGuard处理中排除?