9. Over-simplification
Capacity plans based on current statistics even allowing 100% increases in 12 to 24 months often turn out to be woefully inadequate. Email doesn't always behave like other systems when it comes to capacity planning because a variety of factors can cause sudden increases and because the peak-to-average ratio for network traffic can vary tremendously from one group of users to another.

10. Lack of planning
The absence of capacity planning exacerbates all of these issues and is almost always a central factor in catastrophic failures where systems are down, or highly unreliable, for extended periods of time. To make matters worse, when a system begins to crumble, it's not always clear what the problem is. Tactical fixes are often layered on over time instead of properly planning and undertaking a complete redesign of a company's SMTP infrastructure.

[Don't forget another aspect of email usage: hard disk storage. Because hard drives are so cheap, we tend to encourage our internal users to save all their email for potential future reference. However, over time, email can take up hundreds of megabytes of disk space, requiring upgrades of disk drives. The drives themselves cost little, but the time to perform the upgrade, the downtime of the upgrade system during the upgrade, and the idle time of the user can often have a considerable cost. -- DG]

Number one warning sign
Poor performance is the number one warning sign for Internet email. It's important to remember to test or to attempt to duplicate reported problems such as delivery failures during peak hours.

When a gateway or MTA falls behind in processing messages during peak hours messages queue up faster than they are sent or delivered. When overloaded, the performance of most MTAs and gateways will degrade leading to a rapid reduction in efficiency. The gateway or MTA may catch up slowly during non-peak hours but when users begin complaining that recipients on the net aren't getting messages in a timely manner it's time to start looking for the bottlenecks.

Despite the many issues and risk factors for SMTP, Internet email bottlenecks are often found in the internal email system. In other words, users may complain about slow Internet email but, of course, they don't understand the email system. The actual problem could be in the internal email routing. This should be ruled out before any changes are made or hardware is purchased.

A "peak" at capacity planning
Network planners and technical staff often don't know how to predict email traffic on the network or how to justify needed resources to management. Capacity planning for Internet email involves two fundamental things: server sizing and network engineering. Here are a couple of pointers:

Typically there are two major utilization peaks for a messaging system associated with the business hours of a given group of users within a given time zone. For a company where most employees work from 9 to 5 the peaks tend to occur at approximately 10:30AM and 1:45PM and last as long as 1 hour. Keep in mind that your actual mileage may vary. Note that the apparent length of peaks could be longer for systems that have insufficient capacity. It's also important to predict overlapping peaks across time zones if Internet email services are provided centrally for multiple regions.