I like this part:
Because web connections arent as long lived as IMAP connections, we stayed with Apache for our frontends for a while longer. However weve now switched over to using nginx for our frontend web proxy as well, which has also allowed us to increase the keep-alive timeout for HTTP connections to 5 minutes, which should result in a small perceptible improvement when moving between pages.
The net result of all this is that each frontend proxy server currently maintains over 10,000 simultaneous IMAP, POP, Web & SMTP connections (including many SSL ones) using only about 10% of the available CPU on 3.20GHz Netburst Xeon based CPUs.
nginx seems to have done it right: a small amount of processes each using epoll. If the kernel doesn't waste a whole heap of memory with each open connection, this would scale really well.