Hardware and performance

Obviously this depends on many factors (hardware, configuration, number of users, how many of those are concurrent, etc.). It always comes down to discovering your bottleneck and what type of responsiveness and reliability is expected. With that being said, I have a LAMP box (4G of RAM) running postfix for around 1000 students. By default, I set all student emails as disabled; however, they can update their email address and enable it if they want to receive emails. I wanted to avoid the complaint that Moodle was spamming anyone. The only way to get emails is if you tell Moodle you want them. So with a single server I have found that we average about 1.5G of traffic per day, a quarter of a million hits, an average of over 45 queries per second, and about 3500 emails per week without problem. I would say, start with the Postfix on the server and if you notice a hit to performance then work toward separating out either the database or email servers to another host. In my experience, one good machine seems to be handling things fine (except for the backup which I have on another machine). Happy Moodling!