The easy way -
First you set up a virtual hostname of mail.example.com and configure x # of mail servers. Say mail1.example.com to mail4.example.com.
When users log on to mail.example.com, the load balancer directs the user to any one of the mail servers to verify the log on information. After successfully logging on, users are redirected to the actual server their mail is stored on. While they are logged on, all subsequent requests go directly to their server.
In order to do so:
1. Each Zimbra servers must have a routeable address/name from the outside.
2. You must configure the virtual hostname on the administration console.
3. You must turn on the following localconfig setting on each mail server:
If you wanted to hide that url, you need something that does address rewriting & connection management to correctly route the request - either a proxy or device - some examples: Load_Balancer_Vendors
zmlocalconfig -e zimbra_auth_always_send_refer=true