well you'd just apt-get it again, and i *think* ubuntu, i.e. debian would use exim. so "apt-get install exim" mind you what does the simplest one mean??? what do you want it to actaully DO? if it's just sending stuff out, then actually you'd possibly not want a real SMTP server at all, just a simulation of one, e.g. ssmtp or esmtp.
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