When sizing a computer to use as an e-mail server, a lot of people have misconceptions regarding the hardware required to perform this task well. The constantly increasing performance of computers seems to lead people into thinking that they really need the latest and most buzzword-compliant stuff, even if they only want to handle a few thousand messages per day.
lthough a certain expertise is required to assess the hardware needs for an organization closely, common sense goes a long way. For a company with 100 users, a reasonably high upper limit for the number of messages per day would be 5,000. That would allow each user to send or receive 50 messages every day. Even if we say that each and every message is sent within the eight hours of the working day, on an average, the system will not have to cope with more than 10 messages per minute. Is it reasonable that a modern computer needs more than six seconds to receive and act upon a single e-mail message, often only a few kilobytes in size? No, it is not.
This little back-of-the-envelope exercise is obviously very rough and does not, for example, take into account the fact that messages typically do not arrive uniformly distributed in time, but it is still a pretty good way of estimating.
Let us now take a little deeper look into what to think about when sizing your server. For an SMTP server that does not perform any content scanning (viruses, spam, etc.), the performance is typically not bound by the CPU but by the I/O performance, specifically the seek time of the hard disk(s) and the quality and configuration of the I/O controller. Throwing more CPU horsepower at the problem will not help. Modern computers are relatively better equipped CPU-wise than I/O-wise, so investing in a multiple gigahertz dual-CPU configuration is probably useless. For any reasonably modern 1 GHz-class PC a handful of messages per second is no problem. That load equates to almost 20,000 messages every hour.
Adding content scanning will probably increase the CPU load quite a lot, and the I/O system will also require more power to keep up. Still, one or two messages per second should not place a noticeable load on the system.
What we have been discussing so far is just the SMTP server. All it does is receive messages and deliver them to other hosts or to local mailboxes. When sizing a server, do not forget that people are going to want to read their e-mail too. This service is provided by server software for POP, IMAP, or both. Just like SMTP software, the key requirement is I/O and not CPU. The number of users of the system is by itself an irrelevant figure; what are important are the usage patterns. How often will the users poll their mailboxes? If 100 users poll their mailboxes once every five minutes, on an average there will be one every three seconds. Checking if a mailbox has any new messages takes a fraction of a second, so the burden will not be significant.
These guidelines may appear vague and non-specific, but it is impossible to give exact figures. The performance you can get from a given piece of hardware depends on so many factors that trying to give anything but general guidelines would be misleading. Use common sense and simple back-of-the-envelope calculations; do not buy the fanciest server you can find unless you are sure you really need it, but also do not use any old abandoned desktop machine you can find. Even if the performance of the old desktop machine may suffice, the components may be old and the service agreement or warranty may be out of date.