I just want to share a little tool that we built, is an Authentication Portal for Zimbra Collaboration that uses Zimbra Preauth keys.
Zimbra Preauth Router is a web Authentication portal for Zimbra Collaboration with a twist, and the twist is: works for 0 downtime migrations. ZPR lets you login users in 2 different Zimbra platforms from a single URL portal, only using a
YAML db file.
This blog post explain a little better the motivation behind it: http://blog.itlinux.cl/blog/2015/04/08/zimbra-preauth-router
The tool is in Github: https://github.com/pbruna/zimbra_preauth_router