If one tenth of a million users login, and send or recieve a single message every day, that is 100k messages a day. A machine could easily be kept running continuously to do this work. It should deliver email over NFS, or possibly, it could live in the same backplane as the user disk server.
It should have a network controller seperate from the disk server if it was in the same box, and its network traffice should perhaps even be partitioned from the network file server traffic otherwise required.