diff --git a/core/admin/mailu/configuration.py b/core/admin/mailu/configuration.py
index 0c4d0abd..68f818a4 100644
--- a/core/admin/mailu/configuration.py
+++ b/core/admin/mailu/configuration.py
@@ -81,7 +81,7 @@ class ConfigManager(dict):
 
     def resolve_host(self):
         optional = [item for item in self.OPTIONAL_HOSTS if item in self.config]
-        for item in self.HOSTS + optional:
+        for item in list(self.HOSTS) + optional:
             host = 'HOST_' + item
             self.config[host] = system.resolve_address(self.config[host])