@ConfigurationProperties(locations = "classpath:mail.properties", 
                         ignoreUnknownFields = false, 
                         prefix = "mail")
public class MailProperties { 
	  public static class Smtp {  
	    private boolean auth;  
	    private boolean starttlsEnable;  
	    // ... getters and setters 
	  }
	  @NotBlank private String host;
	  private int port;  
	  private String from; 
	  private String username;
	  private String password; 
	  @NotNull private Smtp smtp; 
	  // ... getters and setters
}