When selecting a non-stable version, add a message this is version is "only for testing".

The stable version is set via the new environment variable stable_version. E.g.
stable_version=1.7

setup/server.py

@@ -53,7 +53,10 @@ def build_app(path):
 
     @app.context_processor
     def app_context():
-        return dict(versions=os.getenv("VERSIONS","master").split(','))
+        return dict(
+            versions=os.getenv("VERSIONS","master").split(','), 
+            stable_version = os.getenv("stable_version", "master")
+        )
 
     prefix_bp = flask.Blueprint(version, __name__)
     prefix_bp.jinja_loader = jinja2.ChoiceLoader([

setup/templates/base.html

@@ -8,13 +8,21 @@
   <h1>Mailu configuration</h1>
   <p>
     Version
-    <select onchange="window.location.href=this.value;" class="btn btn-primary dropdown-toggle">
+    <select id=version_select onchange="window.location.href=this.value;" class="btn btn-primary dropdown-toggle">
       {% for module in versions %}
       <option value="/{{ module }}" {% if module == version %}selected {% endif %}>{{ module }}</option>
       {% endfor %}
     </select>
   </p>
 
+  {% if version != stable_version %}
+  {% call macros.panel("danger", "You have not selected the stable version") %}
+You have not selected the stable version. The stable version is {{ stable_version }}.
+The selected version can be used for testing and reporting bugs.
+For production scenarios we recommend to use the stable version.
+  {% endcall %}
+  {% endif %}
+
   {% block page %}{% endblock %}
 
 </div>