Each service machine in maths has a full local MMDF installation. However, things are setup so that we have a single primary mail gateway (which is currently salmon). All incoming mail to maths is directed (via DNS MX records) to the mail gateway. Most outgoing mail (no matter what machine it is generated on) goes out via the gateway. The gateway machine is the only machine that uses DNS and sends non-local SMTP mail.
This setup has a number of advantages. There is only one machine which needs close monitoring and only one that needs careful configuration. Most important logging is on one machine. We also have the option of installing smaller, simpler, mail systems on the other machines.
Mail for local users is handled using automatically generated aliases. This means that no matter where (or how) a single user is mailed, all their mail ends up going to a single machine. This is important when one comes to consider mail box locking, NFS, and so on. Local user mail is always delivered directly into that users home directory (under the name newmail), unless overriden using a .maildelivery file.
Up to toplevel, on to MMDF Tailor File.