--- a/src/OptionalConfiguration.hs
+++ b/src/OptionalConfiguration.hs
@@ -31,2 +31,3 @@ import System.IO.Error ( catchIOError )
 import MxList ( MxList(..) )
+import qualified Data.Semigroup as S
 
@@ -59,2 +60,19 @@ merge_maybes (Just _) (Just y) = Just y
 
+instance S.Semigroup OptionalConfiguration where
+  cfg1 <> cfg2 =
+    OptionalConfiguration
+      (merge_maybes (database cfg1) (database cfg2))
+      (merge_maybes (domain_query cfg1) (domain_query cfg2))
+      all_excluded_mx
+      (merge_maybes (forward_query cfg1) (forward_query cfg2))
+      (merge_maybes (host cfg1) (host cfg2))
+      (merge_maybes (password cfg1) (password cfg2))
+      (merge_maybes (port cfg1) (port cfg2))
+      (merge_maybes (username cfg1) (username cfg2))
+    where
+      -- Use only the latter exclude_mx if there are any.
+      all_excluded_mx =
+        exclude_mx $ if (null (get_mxs $ exclude_mx cfg2))
+                     then cfg1
+                     else cfg2