Mail Attributes retrieved as String even though set as Boolean

I set a boolean attribute to a Mail:

boolean smartHostSSL = false;
mail.setAttribute(SMARTHOSTSSL, smartHostSSL);

and then I retireve it in another method:
boolean smartHostSSL = (Boolean)mail.getAttribute(SMARTHOSTSSL);

and there's an exception on this very line:
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean