email port listener attachment processing

I have an email port configured to receive pdf file as an attachment.when email is received to this port specified service is invoked However it gives me following error:
Error Message : javax.mail.MessagingException: MIME part of type "multipart/alternative;
boundary=000_36373821BD9B53499BDF1B204E027E0829F1FC7EB2exch2x7; charset=utf-8" contains object of type instead of MimeMultipart

Following are my settings
Invoke service for each part of multipart message: Yes
Include email headers when passing message to content handler: No
I would appreciate if someone can assist me.