diff --git a/packages/server/groupware/mta-sts.nix b/packages/server/groupware/mta-sts.nix
index 7e692a5..41ea0ba 100644
--- a/packages/server/groupware/mta-sts.nix
+++ b/packages/server/groupware/mta-sts.nix
@@ -22,12 +22,12 @@ in
           pkgs.runCommand "testdir" { } ''
             mkdir -p "$out/.well-known"
 
-            echo "
-              version: STSv1
-              mode: enforce
-              max_age: 604800
-              mx: ${tlsDomain}
-            " > "$out/.well-known/mta-sts.txt"
+            printf "%s\n" \
+              "version: STSv1" \
+              "mode: enforce" \
+              "max_age: 604800" \
+              "mx: ${tlsDomain}"
+              > "$out/.well-known/mta-sts.txt"
           ''
         }
       '';