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.