Hi,
you definitely have to decide which user should get as express and which is not. You can maintain in ztable.
Then as Rick suggested previously , collect the entries from ztable in activity step, add condition step, in one branch use mail step with express and other brach of condition use mail step without express.
Note: This technique only will help if you want to use mail step for one workflow or some workflow step. If you want that user will not get any of the express mail from any workflow, then it will not help.