diff --git a/flake-module.nix b/flake-module.nix
index 7bc32ef..962d08b 100644
--- a/flake-module.nix
+++ b/flake-module.nix
@@ -1,4 +1,8 @@
-{inputs, ...}: {
+{
+  inputs,
+  lib,
+  ...
+}: {
   # debug = true;
   # We only define machines config in this flake yet, so we only include
   # the module that builds these. This file might get fuller, if we need to
@@ -55,8 +59,41 @@
       config.permittedInsecurePackages = ["jitsi-meet-1.0.8043"];
 
       overlays = [
-        (_: _: {
+        (_: prev: {
           alias-to-sieve = inputs.alias-to-sieve.packages.x86_64-linux.default; # add custom package to convert alias files to sieve scripts on the stalwart machine
+          stalwart-mail = assert lib.assertMsg (prev.stalwart-mail.version == "0.11.6-unstable-2025-02-04") ''
+            1. If the bug https://github.com/stalwartlabs/sieve/issues/11 is resolved to our satisfaction, try to remove this overlay.
+            2. Check whether sieve-rs recieved new updates that our patch needs to be rebased upon.
+            3. Check whether the sieve-rs patch below that exchanges the dependency needs updates.
+            4. Then update the version comparison in this assert statement.
+            5. Update the `cargoHash` by setting it to `lib.fakeHash` and then inserting the result.
+            6. Check if we can get rid of alias-to-sieve when stalwart-mail has a builtin mail-forwarding feature.
+          '';
+            # This override encapsulates the buildRustPackage function in a function that takes the arguments given by the nixpkgs-version of stalwart-mail
+            # and adds our patch to the arguments before handing the new arguments to buildRustPackage.
+            prev.stalwart-mail.override (
+              let
+                rp = pkgs.rustPlatform;
+              in {
+                rustPlatform =
+                  rp
+                  // {
+                    buildRustPackage = args:
+                      rp.buildRustPackage (
+                        args
+                        // rec {
+                          cargoPatches = [
+                            # Let stalwart-mail depend on our own version of the sieve crate that does not have some “loop prevention” that disables redirects to the original sender.
+                            # We want to redirect to sender if we simulate alias files and not doing so barely prevents loops in my opinion.
+                            ./patches/sieve-rs.patch
+                          ];
+                          # Replace the string with `lib.fakeHash` after version changes in order to get the new hash value.
+                          cargoHash = "sha256-gb2oFlVA/vE6DoWWW8SCFA3l7rtV2RuosPUY+6IcKNM=";
+                        }
+                      );
+                  };
+              }
+            );
         })
       ];
     };
diff --git a/nixos/modules/mail.nix b/nixos/modules/mail.nix
index 77b058a..f1861a2 100644
--- a/nixos/modules/mail.nix
+++ b/nixos/modules/mail.nix
@@ -1,4 +1,8 @@
 /*
+* Building: We patch our version of stalwart and thus need to built it locally.
+*   Be aware that this needs some hours, about 12Gb RAM and a few Gb free space in /tmp.
+*   If you only want to deploy configuration changes and no software updates, consider building on the target VM.
+*   It has stalwart in its nix store and does not need to rebuild it.
 * Forwarding mails: Update the Sops-secrets in the machine directory, rebuild on the VM and deploy.
 *   Everything else should happen automatically but new redirects might take up to two hours due HRZ infrastructure.
 * Using the web admin interface: Set your SSH to do portforwarding of some local port to port 80 of the VM and
diff --git a/patches/sieve-rs.patch b/patches/sieve-rs.patch
new file mode 100644
index 0000000..b706d03
--- /dev/null
+++ b/patches/sieve-rs.patch
@@ -0,0 +1,27 @@
+diff --git a/Cargo.lock b/Cargo.lock
+index be36759b..b4316639 100644
+--- a/Cargo.lock
++++ b/Cargo.lock
+@@ -6404,8 +6404,7 @@ checksum = "0fda2ff0d084019ba4d7c6f371c95d8fd75ce3524c3cb8fb653a3023f6323e64"
+ [[package]]
+ name = "sieve-rs"
+ version = "0.6.0"
+-source = "registry+https://github.com/rust-lang/crates.io-index"
+-checksum = "15ac54053752c25a0e545dd1953de716abcc80b12cfe0b6c2f2c1c73759d4f45"
++source = "git+https://gitea.mathebau.de/fachschaft/sieve-rs.git#71324550504c0f84fe3e814d05cbe22f90a3b228"
+ dependencies = [
+  "ahash 0.8.11",
+  "bincode",
+diff --git a/Cargo.toml b/Cargo.toml
+index f055474f..2b64c9ac 100644
+--- a/Cargo.toml
++++ b/Cargo.toml
+@@ -63,3 +63,7 @@ incremental = false
+ debug-assertions = false
+ overflow-checks = false
+ rpath = false
++
++
++[patch.crates-io]
++sieve-rs = { git = 'https://gitea.mathebau.de/fachschaft/sieve-rs.git' }
+