Define fallback relay

templates/postfix/main.cf.j2

@@ -123,6 +123,10 @@ transport_maps =
 relayhost = {{ postfix_relayhost}}
 {% endif %}
 
+{% if postfix_smtp_fallback_relay is defined %}
+smtp_fallback_relay = {{ postfix_smtp_fallback_relay }}
+{% endif %}
+
 {% if postfix_canonical_maps is defined %}
 canonical_maps = 
 {% for map in postfix_canonical_maps %}