I've created a new module (magnolia-module-enterprise-validation) in which I've implemented a new validator. (This validator is to be shipped separately from the Form module.)

At the moment, there is no straightforward way to show a custom error message upon failed validation for this validator - the default "invalid input" is not appropriate for this validator. I.e., it is not possible for the validator to show a custom error message in this setup:

• Add the validator to config:/modules/form/config/validators as 'enterpriseEmail' (using a bootstrap file)
• Use the validator for a form field by selecting it there.

Showing a custom error message would require unneccessarily invasive changes to the form as a whole.

A workaround is to show the default error message, but add a prominent hint next to the validated field like: "Addresses from free email providers will not be accepted."