Thank you lodac for your point. I can confirm, that this is definitely the problem with Proxy as separate node and Redirect ProxyMailMode (ZCS 8.0.1 OpenSource). Unfortunately, up till now, I could not get hand on getting used to bugzilla, thus sometimes missing huge amount of information. This would be one of tasks for 2013.
So, what I did as a workarround:
I had a spare server hanging without significant load with Apache server. I enabled vhost for http://mail.domain.com and posted the following line (preliminary enabling rewrite module):
I actually where thinking of creating user friendly error message pages and redirects to more explanatory pages any way, so spare web server for such cases were planned. It would be the case to move forward with this idea. So due to this implementation, we have 2 servers in front:
Redirect permanent / https://mail.domain.com/
1. Apache responding to http://mail.domain.com/ with permanent redirect to https://...
2. Zimbra Proxy server with zimbraReverseProxyMailMode https
At the moment I do not see any other issues with this, as redirects work as expected. Glue part for this is firewall configuration with port forward - 80 (HTTP) to Apache server and 443 (HTTPS) to Zimbra proxy server. DNS resolution obviously should work outside Zimbra.
Let's see how far this bug will turn out in future, but this is kind of working example, without significant intervention into Zimbra configuration. Nevertheless, I'd like to hear some other potential side effects or issues in ZCS operations due to this config example. Currently do not see any, besides another server to keep an eye on.