doc/release-notes: add ssmtp removal notice and alternative

GitHub: closes https://github.com/NixOS/nixpkgs/issues/105710

nixos/doc/manual/from_md/release-notes/rl-2205.section.xml

@@ -462,6 +462,44 @@
           kernel messages is handled by systemd since Linux 3.5.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          <literal>pkgs.ssmtp</literal> has been dropped due to the
+          program being unmaintained. <literal>pkgs.msmtp</literal> can
+          be used instead as a substitute <literal>sendmail</literal>
+          implementation. The corresponding options
+          <literal>services.ssmtp.*</literal> have been removed as well.
+          <literal>programs.msmtp.*</literal> can be used instead for an
+          equivalent setup. For example:
+        </para>
+        <programlisting language="bash">
+{
+  # Original ssmtp configuration:
+  services.ssmtp = {
+    enable = true;
+    useTLS = true;
+    useSTARTTLS = true;
+    hostName = &quot;smtp.example:587&quot;;
+    authUser = &quot;someone&quot;;
+    authPassFile = &quot;/secrets/password.txt&quot;;
+  };
+
+  # Equivalent msmtp configuration:
+  programs.msmtp = {
+    enable = true;
+    accounts.default = {
+      tls = true;
+      tls_starttls = true;
+      auth = true;
+      host = &quot;smtp.example&quot;;
+      port = 587;
+      user = &quot;someone&quot;;
+      passwordeval = &quot;cat /secrets/password.txt&quot;;
+    };
+  };
+}
+</programlisting>
+      </listitem>
       <listitem>
         <para>
           <literal>services.kubernetes.addons.dashboard</literal> was

nixos/doc/manual/release-notes/rl-2205.section.md

@@ -150,6 +150,39 @@ In addition to numerous new and upgraded packages, this release has the followin
 - `security.klogd` was removed.  Logging of kernel messages is handled
   by systemd since Linux 3.5.
 
+- `pkgs.ssmtp` has been dropped due to the program being unmaintained.
+  `pkgs.msmtp` can be used instead as a substitute `sendmail` implementation.
+  The corresponding options `services.ssmtp.*` have been removed as well.
+  `programs.msmtp.*` can be used instead for an equivalent setup. For example:
+
+  ```nix
+  {
+    # Original ssmtp configuration:
+    services.ssmtp = {
+      enable = true;
+      useTLS = true;
+      useSTARTTLS = true;
+      hostName = "smtp.example:587";
+      authUser = "someone";
+      authPassFile = "/secrets/password.txt";
+    };
+
+    # Equivalent msmtp configuration:
+    programs.msmtp = {
+      enable = true;
+      accounts.default = {
+        tls = true;
+        tls_starttls = true;
+        auth = true;
+        host = "smtp.example";
+        port = 587;
+        user = "someone";
+        passwordeval = "cat /secrets/password.txt";
+      };
+    };
+  }
+  ```
+
 - `services.kubernetes.addons.dashboard` was removed due to it being an outdated version.
 
 - `services.kubernetes.scheduler.{port,address}` now set `--secure-port` and `--bind-address` instead of `--port` and `--address`, since the former have been deprecated and are no longer functional in kubernetes>=1.23. Ensure that you are not relying on the insecure behaviour before upgrading.