untested code that may just work

core/admin/mailu/internal/views/rspamd.py

@@ -25,3 +25,8 @@ def rspamd_dkim_key(domain_name):
                 }
             )
     return flask.jsonify({'data': {'selectors': selectors}})
+
+@internal.route("/rspamd/local_domains", methods=['GET'])
+def rspamd_local_domains():
+    domains = set(models.Domain.query.all() + models.Alternative.query.all())
+    return '\n'.join(domains)

core/rspamd/conf/whitelist.conf

@@ -0,0 +1,8 @@
+rules {
+        BLACKLIST_ANTISPOOF = {
+            valid_dmarc = true;
+            blacklist = true;
+            domains = "http://{{ ADMIN_ADDRESS }}/internal/rspamd/local_domains";
+            score = 15.0;
+        }
+}