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.
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:)