You can use a simple trick with php's preg_replace function:

preg_replace('/@example\..*/', '@domain.com', $faker->unique()->safeEmail)

so your laravel model factory might looks like this:

$factory->define(App\User::class, function (Faker\Generator $faker) {
    static $password;
    return [
        'name' => $faker->name,
        'email' => preg_replace('/@example\..*/', '@domain.com', $faker->unique()->safeEmail),
        'password' => $password ?: $password = bcrypt('secret'),
        'avatar' => $faker->imageUrl,
        'remember_token' => str_random(10),
    ];
});

You can use the Faker\Generator to create an auxiliary variables:

$factory->define(App\User::class, function (Faker\Generator $faker) {
    $userName = $faker->userName;
    $uniqueSuffix = $faker->unique()->word;
    $domain = 'your.domain.com';
    $uniqueFakeEmail = "$userName.$uniqueSuffix@$domain";

    return [
        'email' => $uniqueFakeEmail,
        // ...
    ];
});