the suggestion is that we turn off the local mail delivery at the server

I'm assuming you don't know which MTA you're using. On the command line, run telnet localhost 25. It should come back with a 220 header. This would generally tell you the MTA you're using (hit cntl-] and type close to exit):

• If port 25 fails, you're not running an MTA.
• If it works, look for the name, and try service <name> stop.

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To disable its startup, look for a corresponding script in /etc/init.d and delete it. update-rc.d <name> remove may work, too, but I understand update-rc.d enough yet to know.