As zrail mentioned below, it's not that much traffic from a load perspective. It's a Java app, so it uses some RAM, but honestly the hardest load to tune for was the inbound spam.
I have a massively overspecced server that I use for this, and many other sites/projects - 24 GB RAM, dual X5670 CPUs, etc... Typically runs at a load average of only 0.5-0.6. Like I said, overspecced:)
You probably need a better mail server that can more easily handle that load. Check Haraka out (shameless plug, but it's designed for high loads and this kind of app).
FYI: searching for Haraka on duckduckgo links to the page with a description text of "This is the meta description of your form. You can use it for SEO purposes.". Whoever runs the website should probably clean up the meta-tags.
That's approximately 2 hits per second. If you're serving static content (or heavily cached) you could handle that with a Raspberry PI. Heck you could probably do it with an arduino.
Edit: this depends heavily on your traffic pattern, of course. Super bursty traffic from Reddit, TechCrunch, etc is going to swamp your IO, but CPU will never dominate the equation.
Depends on your users I think. If 50% of that traffic is during a 10 hour window of time each day, that could be around 70 views per second. And pulling email for an account could be more costly than a static page view. Then if he's getting that many views, he could be processing tens or hundreds of emails a day per account more or less.... Could get expensive.
> If 50% of that traffic is during a 10 hour window of time each day, that could be around 70 views per second.
How do you get that? Assuming 4M/month, that works out to 130K/day. Even if all that traffic comes in a 10-hour window, that works out to 130,000/36000 =~ 4/sec, not 70/sec.
