Thanks for your reply Jose.
As for your first question, I receive the plaintext packets in port 6666 on the server side. I print it out by tcpdump -A. About the second question, I think adding client=yes to sender's config file and client=no to receiver's config file is not helpful. I tried this before and nothing changes.
Also, I am kind of confused how to run stunnel between server and client. Currently what I am doing is run command "sudo stunnel stunnel.config" in my server side, and run another "sudo stunnel stunnel.config" with slight difference in the configuration file. Is that correct?