This bug has been introduced at Revision 4577 in mailgw.py line 707

        if self.mailgw.default_class:
	            attempts.append(self.default_class)
	else:
	            attempts.append(self.config['MAILGW_DEFAULT_CLASS'])


should be replaced by 
        if self.mailgw.default_class:
	            attempts.append(self.mailgw.default_class)
	else:
	            attempts.append(self.config['MAILGW_DEFAULT_CLASS'])