Add conditional otherwise it fails if there is no

postfix_domains dictionary

templates/postfix/conf.d/transport_relay.j2

@@ -1,5 +1,6 @@
 ### {{ ansible_managed }}
 
+{% if postfix_domains is defined %}
 {% for domain in postfix_domains %}
 {% if domain.type == "relay" %}
 {% if domain.comment is defined %}
@@ -8,3 +9,4 @@
 {{ "%-50s %s" | format(domain.name, domain.relay_host) }}
 {% endif %}
 {% endfor %}
+{% endif %}