title: What is the meaning of java.net.UnknownHostException: not-existing?
body: <p>I've this exception while trying to send an email through an IDM application:</p>
  
  <pre><code>javax.mail.MessagingException: Connection error (java.net.UnknownHostException: not-existing)
      at org.apache.geronimo.javamail.transport.smtp.SMTPConnection.protocolConnect(SMTPConnection.java:161) ~[geronimo-javamail_1.4_provider-1.8.3.jar:1.8.3]
      at org.apache.geronimo.javamail.transport.smtp.SMTPTransport.protocolConnect(SMTPTransport.java:165) ~[geronimo-javamail_1.4_provider-1.8.3.jar:1.8.3]
      at javax.mail.Service.connect(Service.java:251) ~[geronimo-javamail_1.4_spec-1.7.1.jar:1.7.1]
      at org.springframework.mail.javamail.JavaMailSenderImpl.doSend(JavaMailSenderImpl.java:389) ~[spring-context-support-3.2.2.RELEASE.jar:3.2.2.RELEASE]
      ... 74 common frames omitted
  Caused by: java.net.UnknownHostException: not-existing
      at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:178) ~[na:1.7.0_21]
      at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:391) ~[na:1.7.0_21]
      at java.net.Socket.connect(Socket.java:579) ~[na:1.7.0_21]
      at java.net.Socket.connect(Socket.java:528) ~[na:1.7.0_21]
      at java.net.Socket.&lt;init&gt;(Socket.java:425) ~[na:1.7.0_21]
      at java.net.Socket.&lt;init&gt;(Socket.java:280) ~[na:1.7.0_21]
      at org.apache.geronimo.javamail.util.MailConnection.getConnectedSocket(MailConnection.java:307) ~[geronimo-javamail_1.4_provider-1.8.3.jar:1.8.3]
      at org.apache.geronimo.javamail.util.MailConnection.getConnection(MailConnection.java:250) ~[geronimo-javamail_1.4_provider-1.8.3.jar:1.8.3]
      at org.apache.geronimo.javamail.transport.smtp.SMTPConnection.protocolConnect(SMTPConnection.java:140) ~[geronimo-javamail_1.4_provider-1.8.3.jar:1.8.3]
      ... 77 common frames omitted
  </code></pre>
  
  <p>Notes:
  I configured a postfix server to receive local messages, but I don't understand the meaning of this exception</p>