The solution I found for making a backup copy of every incoming email to .eml file is creating a processor like this:
public void process(Exchange ex) throws Exception {
javax.mail.Message mailMessage = ex.getIn(org.apache.camel.component.mail.MailMessage.class).getMessage();
File file = new File( "backupDirectory", "DesiredFileName.eml");
DataOutputStream os = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(file)));
mailMessage.writeTo(os);
os.close();
}
This solution copies the whole message, including all headers, body and attachments.