Zimbra send http traffic to https or keeping both in mix

HTTP proxy can support protocol modes for HTTP or HTTPS only, both HTTP and HTTPS, mixed HTTP and HTTPS or HTTPS redirect from HTTP. Redirect is a popular configuration. This configuration must be made to the proxy servers.

HTTPS redirect from HTTP

zmprov ms proxy.server.name zimbraReverseProxyMailMode redirect

HTTP and HTTPS (support both)

zmprov ms proxy.server.name zimbraReverseProxyMailMode both

HTTPS only

zmprov ms proxy.server.name zimbraReverseProxyMailMode https

HTTP only

zmprov ms proxy.server.name zimbraReverseProxyMailMode http

“mixed” will cause only authentication to be sent over HTTPS

zmprov ms proxy.server.name zimbraReverseProxyMailMode mixed

Src:
https://wiki.zimbra.com/wiki/Enabling_Zimbra_Proxy_and_memcached#Protocol_Requirements_Including_HTTPS_Redirect