Actually print multiple value as described in man 5 virtual

postfix/files/mapping.j2

@@ -3,8 +3,9 @@
 {%- macro format_value(key, value) %}
   {#- Some settings, like virtual_alias_maps can take multiple values. Handle this case. -#}
   {%- if value is iterable and value is not string -%}
-    {%- for item in value %}
-{{ key }}	{{ item }}
+{{ key }}	{% for item in value -%}
+    {{ item }}
+    {%- if not loop.last %}, {% endif %}
     {%- endfor %}
   {%- else -%}
 {{ key }}	{{ value }}