Thank you for your reply!
Actually, I fixed the problom in the administration UI page. Even though the testting email function seems still does not work, other email functions are working well such as notification of creating a review request or closing a review requrest.
However, after I saw your post I went into the global_settings.py, I found that the settings in the file seems did not synchronise to the settings I made in the administration UI page. The server host setting is still
SERVER_EMAIL = 'root@localhost'
ps: I have restarted the servers, but the settings in the file don't change.