This can be achieved by changing the value in the Fields area of RE Config. However you may have a reason why you do not want to do that if, for example you have other systems that integrate or create records in RE.
This is a very simple rule but not one that is overly intuitive. It can also be used for primary salutation too.
|1||Add a rule that checks the primary addressee Id on the constituent record||
|2||The criteria are simply that the primary addressee Id is zero. We apply when the record agrees with the criterion so that if the id is 0 i.e. the primary addressee drop down is blank the rule will apply.|