Updating New Domain Extensions in Concrete5

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I have a client on a 5.6 site that is trying to add a user with an email address that ends with .solutions. Concrete5 is rejecting the change as an invalid email address, although it is valid. I've looked around in the documentation and in the forms and haven't found anything for this. Where in the code is the list of approved domain extensions so I can update his site?

pixo