All hosts running the CLC must run a mail transport agent server
(MTA) on port 25. Eucalyptus uses the MTA to deliver or relay
email messages to cloud users' email addresses. You can use
Sendmail, Exim, postfix, or something simpler. The MTA server
does not have to be able to receive incoming mail.
Many Linux distributions satisfy this requirement with their
default MTA. For details about configuring your MTA, go to the
documentation for your specific product.
To test your mail relay for localhost, send email to yourself
from the terminal using mail.