feat: add In-Reply-To and References headers to email

templates/emails/errors/domain_deleted.html.twig

@@ -0,0 +1,15 @@
+{% extends "../base.html.twig" %}
+
+{% set email_color = '#0056b3' %}
+{% set title = 'Domain Watchdog Alert' %}
+{% block content %}
+    <p>Hello, <br/>
+        We are pleased to inform you that a domain name in your watchlist has been detected as deleted from the WHOIS database.<br/>
+        <strong>Domain name:</strong> {{ domain.ldhName }}<br/>
+        <strong>Detection Date:</strong> {{ domain.updatedAt | date("c") }}<br/>
+        <br/>
+        This domain has been identified as deleted and is potentially available for registration until it is registered by another party.<br/><br/>
+        Thank you for your understanding,<br/>
+        Sincerely,<br/>
+    </p>
+{% endblock %}