While going source spelunking, I came across this piece of code in Rails'
ActiveModel:

```ruby
key = "#{key.to_s.camelize}Validator"

begin
  validator = key.include?("::".freeze) ? key.constantize : const_get(key)
rescue NameError
  raise ArgumentError, "Unknown validator: '#{key}'"
end
```
<small>[Rails source: `active_model/validations/validates.rb`][1]</small>

This means that you can namespace your custom validators and use them like this:

```ruby
# lib/internal/email_validator.rb
module Internal
  class EmailValidator
    def validate_each(record, attribute, value)
      return if value.ends_with?('@private_domain.com')

      record.errors.add(attribute, 'not from private domain')
    end
  end
end

# app/models/admin.rb
class Admin < ApplicationRecord
  validates :email, 'internal/email': true
end
```

[1]: https://github.com/rails/rails/blob/375a4143cf5caeb6159b338be824903edfd62836/activemodel/lib/active_model/validations/validates.rb#L116-L122