TECHNOLOGY TO BE USED: DRBD and MySQL

Heartbeat automates all the moving parts and can work as well with the MySQL master-master active/passive solution as well as it can with the MySQL & DRBD solution. It manages the virtual IP address used by the database, directs DRBD to become primary, or relinquish primary duties, mounts the /dev/drbd0 device, and starts/stops MySQL as needed.
DRBD provides an alternative high availability solution for MySQL. By effectively putting a mirrored file system beneath the database, the Linux operating system is thus replicating all of your data without the database even knowing about it.
The advantages are in simplification of management, and elimination of some of the anomalous behavior of MySQL replication .The potential tradeoffs though are some performance impact, both in how your file system will then respond as it has to wait for acknowledgement of writes on remote destinations as well as a performance hit when you failover as the target MySQL database is starting up fresh, and has to warm its cache before performance will equal that on the former primary.
Heartbeat automates all the moving parts, heartbeat can also be used with the native MySQL master-master active/passive replication solution, so it's likely to be in the mix, whatever high availability solution you go with.

Heartbeat brings together in setting up a high availability cluster for MySQL using DRBD. What makes it even better is that it works as well with MySQL standard replication as it does with DRBD so you have options there as well. Lastly, by exposing a virtual IP address and managing the starting and stopping of all the components, heartbeat nicely keeps everything seamless, thus providing a single target virtual database to hit, but multiple underlying server nodes to protect you, eliminating a single point of failure.

TECHNOLOGY TO BE USED: DRBD and MySQL

Most all firewalls/routers have NAT capabilities built-in that allows you to create maps between the internet and your computer network - this is called port forwarding.
You configure the firewall/router to port forward the TCP port number shown on your main user page to the tech support computer. On your main user page, enter and use the TCP port number you prefer or use the default TCP port number already assigned when your account was created.
NAT is built into all the most common Internet Connection Sharing technologies around. Microsoft has built their ICS around it and every Cable/DSL Broadband Router on the market accomplishes its job with NAT.

TECHNOLOGY TO BE USED: DRBD and MySQL

NAT acts as an interpreter between two networks. It sits between the internet and your network as illustrated in the diagram below. The internet is considered the 'public' Internet side and your network is considered the 'private' LAN side. When a computer in the private side request data from the public side (the internet), the NAT device will open a little conduit between your computer and the destination computer. When the public computer returns results from the request, it is passed back through the NAT device to the requesting computer. By this arrangement your Viewer from the Internet public side the private LAN side.

HA cluster with Centralised Storage ( NAS or SAN)

In this case zimbra server will available with both server but store folder ( Mailbox ) will be on NAS or SAN ( Centralized storage device . ha ( High Availability) will handle IP Shifting between primary and secondary server.

Hybrid Scenario

Zimbra OS is not providing calendar sharing and active sync, We can use Microsoft Exchange or Zimbra Network Edition with Zimbra OS for fulfilling the requirement.

This is economical solution, when large no of email accounts are exists. Also called split domain. One domain can be split in two or more servers.