In today’s business environments, we often hear the term ‘high-availability’. Whether a hardware or software solution, we aim for redundancy at all levels, so as to maximize the availability of mission-critical services and operations.


1. Introduction
This white paper discusses a high-availability solution for the AXIGEN Mail Server, using the Heartbeat package with DRBD.

AXIGEN is a proprietary messaging solution while both Heartbeat and DRBD are open source software released under the GNU Public License (GPL). These software packages can be used together to build scalable and highly available integrated cluster messaging applications on the Linux operating system.

Demilitarized zone, used to secure an internal network from external access. You can use Linux firewall to create DMZ easily. There are many different ways to design a network with a DMZ. The basic method is to use a single Linux firewall with 3 Ethernet cards. The following simple example discusses DMZ setup and forwarding public traffic to internal servers.

Consider the following DMZ host with 3 NIC:
[a] eth0 with 192.168.1.1 private IP address - Internal LAN ~ Desktop system
[b] eth1 with 202.54.1.1 public IP address - WAN connected to ISP router
[c] eth2 with 192.168.2.1 private IP address - DMZ connected to Mail / Web / DNS and other private servers

  

Here is a thorough article about emails, put together by Marshall Brain and Tim Crosby. Inside the article:
Introduction to How Email Works; A Simple Email Server; More Complex Servers; The SMTP Server; The POP3 and IMAP Servers; IMAP Problems and Attachments; Free and Paid Email Services; Email Etiquette, and others.

Introduction to How Email Works:
Every day, the citizens of the Internet send each other billions of email messages. If you're online a lot, you yourself may send a dozen or more emails each day without even thinking about it. Obviously, email has become an extremely popular communication tool. Have you ever wondered how email gets from your computer to a friend halfway around the world?

Running your own email server can grant the owner a significant amount of freedom and experience.  It allows you to set up and maintain multiple email accounts and domains with ease and allows for keeping unwanted mail out of your inboxes.

Also, you can create email distribution lists to allow for simple delivery of messages to a predefined list of addresses, or have an unlimited (up to your hard drive size) amount of space to store mail. You also have full control over what types of spam control, virus protection and relay black lists are protecting you email server. You can also determine what types of protocols users can access their email with.

In this article I will assume you are running an email server on a direct Internet broadband connection with a single WAN IP address. I will assume that you have a server set up and understand the basis behind port forwarding and basic networking skills.

DNS Considerations
In order for other email servers on the Internet to know where to deliver mail for your domain, you must make certain entries in your DNS servers. The most important of these is the MX record. The MX records tell the names or IP addresses of the servers that are allowed to handle email for you domain. You can have multiple records of this type as each one must be assigned a weight or authority that determines the order in which each is contacted to receive mail.

Ages ago (in Internet time), when mainframe dinosaurs roamed the Earth, a new approach to computer networking called "client/server" emerged. Client/server proved to be a more cost-effective way to build many types of networks, particularly PC-based LANs running end-user database applications.

Client/server is just one approach to distributed computing. The client/server model has been popular for a long time, but recently peer-to-peer networking has re-emerged as a viable alternative. Other approaches like clustering also have benefits in specific situations.

Client/server is a computational architecture that involves client processes requesting service from server processes. Some of the most popular applications on the Internet follow the client/server design:
    * Email clients
    * FTP (File transfer) clients
    * Web browsers ....

In this article, we will describe some of the most important things that need to be taken into account when designing a large scale messaging setup.

We are referring to large scale setups, not cluster setups; the main difference being that cluster setups involve a high-availability situation, which is not the case with large scale setups that achieve high availability or redundancy through other means than the mail server software or the way the setup is organized and performed.

In today’s fast paced world, the importance of having an e-mail service needs no further explanation. We all use it on a daily basis, relying on it to keep in touch with the surrounding world. However, when the throughput of information becomes so great that a simple system cannot keep up with it, new methods must be deployed to cope with the growing requirements of networks and their users. Clusters fill up the gap between the need for faster, more powerful systems, and the safety, reliability and easy management of the already existing networks.

Here is an article I found on SearchSecurity.com (TechTarget), about how to prevent spam bots from hijacking an enterprise network. I think you will find it interesting and useful:

"Despite Bill Gates' assertion in 2004 that the problem of spam would be solved by now, it's still with us. In fact, it's on the increase. According to recent figures from Symantec, 61% of emails are spam, and almost 90% of emails emanating from some countries are spam.

This article represents a small guide to understanding the DMZ (Demilitarized Zone) concept and the ways to implement it on a server. It also includes useful tips on how to increase the security of such setups, with a special focus on the Linux/*NIX-type systems:

Common setups used for small and medium networks include a firewall that processes all the requests from the internal network (LAN) to the Internet and from the Internet to the LAN. This firewall is the only protection the internal network has in these setups and it handles any NAT (Network Address Translation), forwarding and filtering requests as necessary. In most cases, the firewall also runs public services accessible from the Internet, such as web services and e-mail services. Within such setups, the DMZ is thus installed on and limited, we may say, to the server.