diff --git a/default.nix b/default.nix
index d3de212..97485a4 100644
--- a/default.nix
+++ b/default.nix
@@ -41,6 +41,21 @@ in
   options.mailserver = {
     enable = mkEnableOption "nixos-mailserver";
 
+    enableNixpkgsReleaseCheck = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to check for a release mismatch between NixOS mailserver and
+        Nixpkgs.
+
+        Using mismatched versions is likely to cause compatibility issues
+        and may require migrations that make an eventual rollback tricky.
+
+        It is therefore highly recommended to use a release of NixOS mailserver
+        that corresponds with your chosen release of Nixpkgs.
+      '';
+    };
+
     stateVersion = mkOption {
       type = types.nullOr types.ints.positive;
       default = null;
diff --git a/flake.lock b/flake.lock
index 1224eef..6f64a7b 100644
--- a/flake.lock
+++ b/flake.lock
@@ -93,29 +93,12 @@
         "type": "github"
       }
     },
-    "nixpkgs-25_05": {
-      "locked": {
-        "lastModified": 1761999846,
-        "narHash": "sha256-IYlYnp4O4dzEpL77BD/lj5NnJy2J8qbHkNSFiPBCbqo=",
-        "owner": "NixOS",
-        "repo": "nixpkgs",
-        "rev": "3de8f8d73e35724bf9abef41f1bdbedda1e14a31",
-        "type": "github"
-      },
-      "original": {
-        "owner": "NixOS",
-        "ref": "nixos-25.05",
-        "repo": "nixpkgs",
-        "type": "github"
-      }
-    },
     "root": {
       "inputs": {
         "blobs": "blobs",
         "flake-compat": "flake-compat",
         "git-hooks": "git-hooks",
-        "nixpkgs": "nixpkgs",
-        "nixpkgs-25_05": "nixpkgs-25_05"
+        "nixpkgs": "nixpkgs"
       }
     }
   },
diff --git a/flake.nix b/flake.nix
index b242176..990324e 100644
--- a/flake.nix
+++ b/flake.nix
@@ -13,7 +13,6 @@
       inputs.nixpkgs.follows = "nixpkgs";
     };
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
-    nixpkgs-25_05.url = "github:NixOS/nixpkgs/nixos-25.05";
     blobs = {
       url = "gitlab:simple-nixos-mailserver/blobs";
       flake = false;
@@ -26,7 +25,6 @@
       blobs,
       git-hooks,
       nixpkgs,
-      nixpkgs-25_05,
       ...
     }:
     let
@@ -39,11 +37,6 @@
           nixpkgs = nixpkgs;
           pkgs = nixpkgs.legacyPackages.${system};
         }
-        {
-          name = "25.05";
-          nixpkgs = nixpkgs-25_05;
-          pkgs = nixpkgs-25_05.legacyPackages.${system};
-        }
       ];
       testNames = [
         "clamav"
diff --git a/mail-server/assertions.nix b/mail-server/assertions.nix
index c606bda..374c17b 100644
--- a/mail-server/assertions.nix
+++ b/mail-server/assertions.nix
@@ -3,7 +3,34 @@
   lib,
   ...
 }:
+
+let
+  mailserverRelease = "25.11";
+  nixpkgsRelease = lib.trivial.release;
+  releaseMismatch =
+    config.mailserver.enableNixpkgsReleaseCheck && mailserverRelease != nixpkgsRelease;
+in
+
 {
+  warnings = lib.optional releaseMismatch ''
+    You are using
+
+      NixOS Mailserver version ${mailserverRelease} and
+      Nixpkgs version ${nixpkgsRelease}.
+
+    Using mismatched versions is likely to cause compatibility issues
+    and may require migrations that make an eventual rollback tricky.
+
+    It is therefore highly recommended to use a release of
+    NixOS mailserver that corresponds with your chosen release of Nixpkgs.
+
+    If you insist then you can disable this warning by adding
+
+      mailserver.enableNixpkgsReleaseCheck = false;
+
+    to your configuration.
+  '';
+
   # We guard all assertions by requiring mailserver to be actually enabled
   assertions = lib.optionals config.mailserver.enable (
     [