We have a business rule on Journal.Email that is configured to trigger when Parentlink_Category is set, but only if the Subject field is blank. However, it never triggers, even though new records are created with no Subject.
You will need to set an Initialization Rule for the Subject field that sets with up with a blank/empty field. You can accomplish this by creating a new rule for the field, and then just clicking Save without entering any text in the Expression field. Once saved, it should display as [empty value] in grey. If there is no Initialization Rule set, then the default value is NULL, which will not meet the criteria in the constraint.