Cachet/app/models/Subscriber.php

<?php

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletingTrait;
use Watson\Validating\ValidatingTrait;

class Subscriber extends Model
{
    use SoftDeletingTrait, ValidatingTrait;

    /**
     * The validation rules.
     *
     * @var string[]
     */
    protected $rules = [
        'email' => 'required|email',
    ];

    /**
     * The fillable properties.
     *
     * @var string[]
     */
    protected $fillable = ['email'];
}