When using layer 4 switching the DNS setup is greatly simplified. The switching takes care of multiplexing incoming TCP/IP connections in place of round-robin DNS resolution. The IP address of the server pool in the case of an Alteon Switch[AWS] or the IP address of the Linux Director in the case of the Linux Virtual Server[Zha] should be advertised as the primary MX for the domain. The front end mail servers should comprise the server pool.
If using the Linux Virtual Server then having this as a single point of failure can be avoided by having a backup server that can be brought on line using IP address takeover technology as employed by fake[Hor98] and Heartbeat[Rob].