You could add in a check to your workflow where it checks to make sure your email values follow a certain structure - this would typically be a RegEx Match to check for the pattern XYZ@XYZ.ZYZ or whatever pattern you need. If you are talking about checking if the email itself is valid (i.e. if it would bounce back), I am not sure if this is possible because the email has to try to be sent before getting that response back.
