Zimbra server ver. 8.0.3_GA_5664.FOS

The internal web client user is using to access zimbra
The external web client user is using http://zimbra.abc_company.com
I use apache2 proxy-pass to internal zimbra:
        ServerName zimbra.abc_company.com
        ProxyPass /
        ProxyPassReverse /
        ErrorLog ${APACHE_LOG_DIR}/zimbra-error.log
        CustomLog ${APACHE_LOG_DIR}/zimbra-access.log common
and firewall will block tcp:8000 from outside

all operation is work (internal & external). But, when external client click 'Change Password' (right-top corner). The new window will pop up and URL is http://zimbra.abc_company.com:8000/h/changepass , beacuse the tcp:8000 blocked, all external user cannot access this page and cannot change password.

How to fix this problem?
Thanks for help.