I don't know why but this is one of those things that I find extremely annoying to set up. Mainly because it often requires installing a full-blown MTA to do a much simpler job than it was designed to do. Unfortunately, ssmtp and exim, the only simpler ones often suck (ssmtp especially which even the maintainer gave up on). What follows is how I got postfix working on my Feisty machine:
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http://www.davidgrant.ca/setting_up_postfix_to_send_outgoing_mail_on_ubuntu