code: public MailBuilder mail() {
          if (mailBuilder != null)
              return mailBuilder.content(build());
          else
              return MailBuilder.using(configuration).content(build());
      }
docs: Convenience method for chaining the creation of the content body with
  the creation of the mail envelope.
  
  If this builder was created associated to a MailBuilder, that one (with
  the content updated) is returned.
  
  Else it returns a new {@link MailBuilder} with this content and using the
  same * configuration that this Builder was created with.
  
  @return a {@link MailBuilder} with this content