Yup validate is either String or Array of strings

oneOf only works with literal values. Lazy allows you to provide a schema dynamically as shown below

{ 
  email: yup.lazy(val => (Array.isArray(val) ? yup.array().of(yup.string()) : yup.string()))
}

This YUP simple validation work for my case when Form contains multi-select field and keeping this field as mandatory and at least one option is required to select.

 selectOptions: array()
         .min(1, "You can't leave this blank.")
         .required("You can't leave this blank.")
         .nullable()

{
  email: yup.mixed()
    .when('isArray', {
      is: Array.isArray,
      then: yup.array().of(yup.string()),
      otherwise: yup.string(),
    })
}

But a set of checkboxes can produce an array, and text input would not. Are you searching for a solution to validate emails divided by separator?