Email your customers actually receive.

# send one message
curl https://api.sendrill.com/v1/messages \
  -H "X-API-Key: $SENDRILL_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "orders@mail.kallisti.gr",
    "to": ["maria@example.com"],
    "subject": "Your order #4182",
    "html": "<p>Thanks, Maria.</p>",
    "tags": ["order", "shop"]
  }'

# 202 Accepted
# { "messages": [ { "id": "msg_7Kq2…",
#                   "to": "maria@example.com",
#                   "status": "sent" } ] }

use Illuminate\Support\Facades\Http;

Http::withHeaders(['X-API-Key' => config('services.sendrill.key')])
    ->post('https://api.sendrill.com/v1/messages', [
        'from'    => 'orders@mail.kallisti.gr',
        'to'      => [$order->email],
        'subject' => "Your order #{$order->number}",
        'template' => 'order-confirmation',
        'vars'    => ['total' => $order->formattedTotal()],
    ])
    ->throw();

// Suppressed recipients come back as "skipped" — never
// as an error, and never as a second send.

const res = await fetch('https://api.sendrill.com/v1/messages', {
  method: 'POST',
  headers: {
    'X-API-Key': process.env.SENDRILL_KEY,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    from: 'orders@mail.kallisti.gr',
    to: [customer.email],
    subject: `Your order #${order.number}`,
    html: render('order-confirmation', order),
  }),
});

const { messages } = await res.json();