Simple Single-Layer Architecture
In the simplest form, sometimes referred to as messaging in a box, all the components of the messaging server run on a single system (FIGURE 3-1) and no proxy is involved.
Figure 3-1. Messaging Server, Storage, and Firewall Messaging System
The components are:
Directory
MTA
Mailstore
The benefits and drawbacks to this type of architecture are:
Simple
Easy to manage
Easy to troubleshoot
Limited scalability— This architecture is obviously limited in scalability to the computer system's size (CPU and memory) and operating system's scalability. Do not let this fool you into thinking that it cannot scale at all. In some messaging architectures, servers utilizing Sun hardware and the Solaris OE have scaled as high as 16 CPUs and are supporting thousands of concurrent users.
No high availability— With everything in a single server, you have no redundancy beyond what is provided with that single system. You can get some availability through redundant components such as:
Power supplies
Network interfaces
CPU/Memory boards
RAID protected storage
Security— Just because it is a simple configuration does not mean that it is entirely without security or that you cannot secure the system. Standard practices of turning off unused services, for example telnet, and replacing them with alternatives like ssh still apply, as does the use of firewall technology. However, without some form of relay or proxy for SMTP traffic from the Internet, this system will be accessible through SMTP directly from the Internet.
So, while a simple configuration is less secure than other configurations, it is not completely insecure. Overall this simple configuration tends to work for labs, training facilities, and very small systems where simplicity is the foremost requirement.