It isn't logically feasible to have one machine handle all mail delivery since that would place too much importance on that machine. If cappio is down, their backup MX host will go to postoffice. Postoffice sits and queues mail, then, waiting for imsa.edu to come back up. It then ph's for 'user' and sends the message on to the person's 'email' field. Listproc, as will be seen, is an acception and works just like postoffice.
Mail is first priority to deliver to mocha actually and to postoffice as a backup.
Adding a new machine to the titan cluster From a mail standpoint, all you need do is add the host's A record. Then, make an MX record priority 5 pointing to cappio.imsa.edu. The next priority of 10 should point to postoffice.imsa.edu. B. Cavaeates for the future I think our mail firewall should be named imsa.imsa.edu. rather than cappio. This is a change in the extreme 'only if you care' range.