Validations at domain model level are validate at insert or update at database level. so if your data which you POST originate from the database, its validated. If it is entered by a user, but didn't hit the database, no validation has taken place. In these scenarios you should validate in the microflow before POST activity
https://academy.mendix.com/link/modules/95/lectures/770/8.3-Validation-in-Microflows
Sounds like access rights are not set up correctly in the domain model.
If the microflow handling the POST operation is generated by the mendix platform itself, then it will save the request payload automatically into the database. As you said, you had a unique type of constraint on the email id, if you try to duplicate the email and save it into the database then it will throw the 403- forbidden error by default.