Simple Mail System for Ubuntu?

Just do a

sudo dpkg-reconfigure exim4-config

and pick the "mail sent by smarthost; no local mail" option. Now for the options:

• Specify a system mail name, specify to listen for incoming SMTP connections only on port 25 of localhost
• Don't specify any other destionations for which mail is accepted
• Specify subdomain.mybusiness.com as the visible domain name for local users.
• Specify the IP address of your actual mail server -- that's the smarthost. If it requires authentication put the name of the server, your username and password in /etc/exim4/passwd.client

There you go: a complete mail setup for Debian/Ubuntu in under one minute!

If you're just sending out mail, you might be able to give nullmailer a try, it's in the Ubuntu extras repository on Jaunty (aptitude "show" output):

Package: nullmailer
Description: simple relay-only mail transport agent
 Nullmailer is a replacement MTA for hosts, which relay to a fixed set of smart
 relays. It is designed to be simple to configure and especially useful on slave
 machines and in chroots. 

 The sendmail interface of this package doesn't provide the -bs switch, hence
 it's not LSB compatible!
Homepage: http://untroubled.org/nullmailer/

Link to the project homepage: here

You will still require a valid SMTP server to be used with ssmtp. Once you configure ssmtp to talk to the up-stream SMTP server, it will relay all your email through that server. This will usually be your ISP server or 3rd party SMTP server.