New flexible configuration features for postfix

• allow lists when configuring virtual alias destinations
• allow DNS entries for postscreen network config (resolves MX to v4 and v6)

postfix/templates/postscreen_access.cidr.j2

@@ -12,5 +12,22 @@
 #
 
 {% for net in postfix_network_access %}
+{% if 'comment' in net %}
+{{ net.comment|comment('plain', prefix='', postfix='') }}
+{% endif %}
+{% if 'cidr' in net %}
 {{ net.cidr }}	{{ net.action|default('dunno') }}
+{% endif %}
+{% if 'mx' in net %}
+{{ net.mx|comment('plain', prefix='', postfix='') }}
+{% for mx in q('dig', net.mx, 'qtype=MX') %}
+{% for addr in q('dig', mx.split(' ', maxsplit=1)[-1], 'qtype=A') %}
+{{ addr }}	{{ net.action|default('dunno') }}
+{% endfor %}
+{% for addr in q('dig', mx.split(' ', maxsplit=1)[-1], 'qtype=AAAA') %}
+{{ addr }}	{{ net.action|default('dunno') }}
+{% endfor %}
+{% endfor %}
+{% endif %}
+
 {% endfor %}