Deprecate borgbackup, rsnapshot, monit integrations

The plan is to start warning now with the intent to remove these
integrations before the 26.11 release.

mail-server/assertions.nix

@@ -12,24 +12,55 @@ let
 in
 
 {
-  warnings = lib.optional releaseMismatch ''
-    You are using
+  warnings =
+    lib.optionals releaseMismatch [
+      ''
+        You are using
 
-      NixOS Mailserver version ${mailserverRelease} and
-      Nixpkgs version ${nixpkgsRelease}.
+          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.
+        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.
+        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
+        If you insist then you can disable this warning by adding
 
-      mailserver.enableNixpkgsReleaseCheck = false;
+          mailserver.enableNixpkgsReleaseCheck = false;
 
-    to your configuration.
-  '';
+        to your configuration.
+      ''
+    ]
+    ++ lib.optionals config.mailserver.borgbackup.enable [
+      ''
+        `mailserver.borgbackup` will be removed after 26.05.
+
+        The borgbackup integration will be removed with the recommendation to
+        migrate to the upstream `services.borgbackup` module, which receives far
+        superior maintenance and testing.
+
+        NixOS manual: https://nixos.org/manual/nixos/stable/#module-borgbase
+      ''
+    ]
+    ++ lib.optionals config.mailserver.backup.enable [
+      ''
+        `mailserver.backup` will be removed after 26.05.
+
+        The rsnapshot integration will be removed due to lack of maintenance,
+        expertise and tests to make sure it still works. Please use the upstream
+        module directly instead.
+      ''
+    ]
+    ++ lib.optionals config.mailserver.monitoring.enable [
+      ''
+        `mailserver.monitoring` will be removed after 26.05.
+
+        The monit integration will be removed due to lack of maintenance,
+        expertise and tests to make sure it still works.
+      ''
+    ];
 
   # We guard all assertions by requiring mailserver to be actually enabled
   assertions = lib.optionals config.mailserver.enable (