diff --git a/packages/server/groupware/mta-sts.nix b/packages/server/groupware/mta-sts.nix
index 385f688..0bcf410 100644
--- a/packages/server/groupware/mta-sts.nix
+++ b/packages/server/groupware/mta-sts.nix
@@ -3,6 +3,7 @@
 { pkgs, flakeLib, ... }:
 
 let
+  mailDomain = "mx.${domain}";
   tlsDomain = "mta-sts.${domain}";
 in
 {
@@ -26,7 +27,7 @@ in
               "version: STSv1" \
               "mode: enforce" \
               "max_age: 604800" \
-              "mx: ${tlsDomain}" \
+              "mx: ${mailDomain}" \
               > "$out/.well-known/mta-sts.txt"
           ''
         }