Check release version compat

To move into a better position to align this project with nixpkgs
unstable breaking changes we now default to require a matching nixpkgs
release.

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;