In case you are using ubuntu 9.10, this is my notes to set up the email relay to exchange:
sudo apt-get install mailx
Select sattellite and enter relay server (in this case the exchange server)
test in terminal
(NB: company server only accept relay for local addresses)
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Edit the Nagios email notification commands found in /usr/local/nagios/etc/objects/commands.cfg and change any '/bin/mail' references to '/usr/bin/mail'. Once you do that you'll need to restart Nagios to make the configuration changes live.