Add support for sender rewriting using postsrsd

With SRS we support forwarding of mails without (fully) breaking SPF
alignment.

README.md

@@ -57,6 +57,8 @@ SNM branch corresponding to your NixOS version.
 * User Aliases
   * [x] Regular aliases
   * [x] Catch all aliases
+* Improve the Forwarding Experience
+  * [x] [Sender Rewriting Scheme](https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme)
 
 ### In the future
 
@@ -69,7 +71,6 @@ SNM branch corresponding to your NixOS version.
   * [ ] Allow passing DKIM signing keys
 * Improve the Forwarding Experience
   * [ ] Support [ARC](https://en.wikipedia.org/wiki/Authenticated_Received_Chain) signing with [Rspamd](https://rspamd.com/doc/modules/arc.html)
-  * [ ] Support [SRS](https://en.wikipedia.org/wiki/Sender_Rewriting_Scheme) with [postsrsd](https://github.com/roehling/postsrsd)
 * User management
   * [ ] Allow local and LDAP user to coexist
 * OpenID Connect

default.nix

@@ -1101,6 +1101,28 @@ in
       '';
     };
 
+    srs = {
+      enable = mkEnableOption "Sender Rewrite Scheme";
+
+      domain = mkOption {
+        type = with types; nullOr str;
+        default = config.mailserver.systemDomain;
+        defaultText = literalExpression "config.mailserver.systemDomain";
+        example = "srs.example.com";
+        description = ''
+          Mail domain used for ephemeral SRS envelope addresses.
+
+          :::{note}
+          This domain can only support relaxed SPF alignment.
+          :::
+
+          :::{important}
+          For privacy reasons you should use a dedicated domain when serving multiple unrelated domains.
+          :::
+        '';
+      };
+    };
+
     redis = {
       configureLocally = mkOption {
         type = types.bool;

docs/index.rst

@@ -23,16 +23,22 @@ Welcome to NixOS Mailserver's documentation!
 
 .. toctree::
    :maxdepth: 1
+   :caption: Features
+
+   fts
+   ldap
+   srs
+
+.. toctree::
+   :maxdepth: 0
    :caption: How-to
 
    backup-guide
    add-radicale
    add-roundcube
    rspamd-tuning
-   fts
    flakes
    autodiscovery
-   ldap
 
 Indices and tables
 ==================

docs/release-notes.rst

@@ -7,6 +7,8 @@ NixOS 25.11
 - The ``systemName`` and ``systemDomain`` options have been introduced to have
   reusable configurations for automated reports (DMARC, TLSRPT). They come with
   reasonable defaults, but it is suggested to check and change them as needed.
+- Support for the `Sender Rewriting Scheme`_ has been added, which allows
+  forwarding mail without breaking SPF by rewriting the envelope address.
 - The default key length for new DKIM RSA keys was increased to 2048 bits as
   recommended in `RFC 8301 3.2`_.
   We recommend rotating existing keys, as the RFC advises that signatures from
@@ -29,6 +31,7 @@ NixOS 25.11
   `tlsrpt-reporter`_. They can be enabled with the ``mailserver.tlsrpt.enable``
   option.
 
+.. _Sender Rewriting Scheme: srs.html
 .. _RFC 8301 3.2: https://www.rfc-editor.org/rfc/rfc8301#section-3.2
 .. _RFC 8314 3.3: https://www.rfc-editor.org/rfc/rfc8314#section-3.3
 .. _RFC 8314 4.1: https://www.rfc-editor.org/rfc/rfc8314#section-4.1

docs/srs.rst

@@ -0,0 +1,102 @@
+Sender Rewriting Scheme
+=======================
+
+The Sender Rewriting Scheme (SRS) allows mail servers to forward emails without
+breaking SPF checks. By rewriting the envelope sender to an address within the
+forwarder’s domain, SRS ensures that forwarded messages pass SPF validation,
+preventing them from being rejected as spoofed or unauthorized.
+
+How SRS works in practice
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+1. ``alice@foo.example`` receives an E-Mail from ``bob@bar.example``. Both the
+envelope sender as well as the ``From`` header show ``bob@bar.example``. This
+results in strict SPF alignment, because ``bar.example`` is the domain used in
+both the ``Return-Path`` and ``FROM`` headers.
+
+2. ``alice@foo.example`` forwards the mail to ``charlie@moo.example`` and
+uses SRS to rewrite the envelope sender to originate from the local SRS domain
+(e.g. `SRS0=HHH=TT=bar.example=alice@foo.example`). The ``FROM`` header remains
+unchanged. This ensures that the forwarded mail succeeds SPF checks.
+
+3. The email reaches ``charlie@moo.example``. SPF passes because the sender
+domain in the envelope has been rewritten. The mismatch between envelope sender
+domain and ``FROM`` domain does however break strict SPF alignment.
+
+Enabling SRS
+~~~~~~~~~~~~
+
+In a simple setup just enabling SRS will use your ``mailserver.systemDomain``
+when rewriting the envelope sender domain.
+
+.. code:: nix
+
+  {
+    mailserver = {
+      srs = {
+        enable = true;
+        #domain = "srs.example.com";
+      };
+    };
+  };
+..
+
+While you can reuse an existing email domain for SRS, it is recommended to
+configure a dedicated SRS domain. This is particularly important under the
+following conditions:
+
+* Multiple unrelated mail domains are hosted on the mailserver
+* The mail domain requires strict SPF alignment in its DMARC policy
+
+Required DNS changes
+~~~~~~~~~~~~~~~~~~~~
+
+.. note::
+  In the following example we assume that you want to set up a dedicated SRS
+  domain. If that is not the case you already have SPF and DKIM set up for the
+  system domain. If you have a DMARC record on the system domain, make sure it
+  uses a relaxed SPF alignment policy (``aspf=r``).
+
+First we set up an MX record. This is so that we can receive and route bounces
+that can result from forwards.
+
+======================== ===== ==== ======== =====================
+Name (Subdomain)         TTL   Type Priority Value
+======================== ===== ==== ======== =====================
+srs.example.com          10800 MX   10       ``mail.example.com``
+======================== ===== ==== ==============================
+
+Next up is the SPF record on the SRS domain to allow SPF authentication.
+
+======================== ===== ==== ===================
+Name (Subdomain)         TTL   Type Value
+======================== ===== ==== ===================
+srs.example.com          10800 TXT  ``v=spf1 mx -all``
+======================== ===== ==== ===================
+
+Then we deploy the DKIM record with the `p=<value>` taken from
+``/var/dkim/srs.example.com.mail.txt``, that appears after deploying with SRS
+enabled.
+
+=============================== ===== ==== ========================================
+Name (Subdomain)                TTL   Type Value
+=============================== ===== ==== ========================================
+mail._domainkey.srs.example.com 10800 TXT  ``v=DKIM1; k=rsa; p=<really-long-key>``
+=============================== ===== ==== ========================================
+
+Finally we can tie this together in the DMARC record to require receivers to
+verify the requested SPF/DKIM alignment.
+
+.. note::
+
+   The SRS domain can only support relaxed SPF alignment due to the envelope
+   sender and ``FROM`` header mismatch.
+
+======================== ===== ==== =========================================
+Name (Subdomain)         TTL   Type Value
+======================== ===== ==== =========================================
+_dmarc.srs.example.com   10800 TXT  ``v=DMARC1; p=reject; aspf=r; adkim=s;``
+======================== ===== ==== =========================================
+
+We can safely configure a ``reject`` policy on the SRS domain, to enforce the
+SPF and DKIM alignment as configured above.

flake.nix

@@ -84,6 +84,7 @@
                 _module.check = false;
                 mailserver = {
                   fqdn = "mx.example.com";
+                  systemDomain = "example.com";
                   domains = [
                     "example.com"
                   ];

mail-server/postfix.nix

@@ -261,6 +261,24 @@ in
       configurePostfix = true;
     };
 
+    # Sender Rewriting Scheme (https://www.libsrs2.net/srs/srs.pdf)
+    services.postsrsd = {
+      inherit (cfg.srs) enable;
+      configurePostfix = true;
+      settings = {
+        domains = lib.unique (
+          [
+            cfg.fqdn
+            cfg.sendingFqdn
+            cfg.systemDomain
+          ]
+          ++ cfg.domains
+        );
+        separator = "=";
+        srs-domain = cfg.srs.domain;
+      };
+    };
+
     systemd.services.postfix-setup = lib.mkIf cfg.ldap.enable {
       preStart = ''
         ${appendPwdInVirtualMailboxMap}

mail-server/rspamd.nix

@@ -50,6 +50,8 @@ let
         echo "Generated key for domain ${domain} and selector ${cfg.dkimSelector}"
       fi
     '';
+
+  dkimDomains = lib.unique (cfg.domains ++ (lib.optionals cfg.srs.enable [ cfg.srs.domain ]));
 in
 {
   config = lib.mkIf cfg.enable {
@@ -201,7 +203,7 @@ in
           SupplementaryGroups = [ config.services.redis.servers.rspamd.group ];
         }
         (lib.optionalAttrs cfg.dkimSigning {
-          ExecStartPre = map createDkimKeypair cfg.domains;
+          ExecStartPre = map createDkimKeypair dkimDomains;
           ReadWritePaths = [ cfg.dkimKeyDirectory ];
         })
       ];

scripts/generate-options.py

@@ -27,6 +27,7 @@ groups = [
     "mailserver.loginAccounts",
     "mailserver.certificate",
     "mailserver.dkim",
+    "mailserver.srs",
     "mailserver.dmarcReporting",
     "mailserver.fullTextSearch",
     "mailserver.redis",