nixos/mailman: make Postfix support optional (provided you configure the MTA yourself)

Mailman can now work with MTAs other than Postfix. You'll have to configure
it yourself using the options in `services.mailman.settings.mta`.

This addition is reflected in the release notes for 21.03.

nixos/doc/manual/release-notes/rl-2103.xml

@@ -250,6 +250,19 @@
   <title>Other Notable Changes</title>
 
   <itemizedlist>
+   <listitem>
+    <para>
+     The Mailman NixOS module (<literal>services.mailman</literal>) has a new
+     option <xref linkend="opt-services.mailman.enablePostfix" />, defaulting
+     to true, that controls integration with Postfix.
+    </para>
+    <para>
+     If this option is disabled, default MTA config becomes not set and you
+     should set the options in <literal>services.mailman.settings.mta</literal>
+     according to the desired configuration as described in
+     <link xlink:href="https://mailman.readthedocs.io/en/latest/src/mailman/docs/mta.html">Mailman documentation</link>.
+    </para>
+   </listitem>
    <listitem>
     <para>
      The default-version of <literal>nextcloud</literal> is <package>nextcloud20</package>.

nixos/modules/services/mail/mailman.nix

@@ -38,7 +38,7 @@ let
   webSettingsJSON = pkgs.writeText "settings.json" (builtins.toJSON webSettings);
 
   # TODO: Should this be RFC42-ised so that users can set additional options without modifying the module?
-  mtaConfig = pkgs.writeText "mailman-postfix.cfg" ''
+  postfixMtaConfig = pkgs.writeText "mailman-postfix.cfg" ''
     [postfix]
     postmap_command: ${pkgs.postfix}/bin/postmap
     transport_file_type: hash
@@ -81,7 +81,7 @@ in {
       enable = mkOption {
         type = types.bool;
         default = false;
-        description = "Enable Mailman on this host. Requires an active Postfix installation.";
+        description = "Enable Mailman on this host. Requires an active MTA on the host (e.g. Postfix).";
       };
 
       package = mkOption {
@@ -92,6 +92,20 @@ in {
         description = "Mailman package to use";
       };
 
+      enablePostfix = mkOption {
+        type = types.bool;
+        default = true;
+        example = false;
+        description = ''
+          Enable Postfix integration. Requires an active Postfix installation.
+
+          If you want to use another MTA, set this option to false and configure
+          settings in services.mailman.settings.mta.
+
+          Refer to the Mailman manual for more info.
+        '';
+      };
+
       siteOwner = mkOption {
         type = types.str;
         example = "postmaster@example.org";
@@ -182,7 +196,7 @@ in {
         pid_file = "/run/mailman/master.pid";
       };
 
-      mta.configuration = lib.mkDefault "${mtaConfig}";
+      mta.configuration = lib.mkDefault (if cfg.enablePostfix then "${postfixMtaConfig}" else throw "When Mailman Postfix integration is disabled, set `services.mailman.settings.mta.configuration` to the path of the config file required to integrate with your MTA.");
 
       "archiver.hyperkitty" = lib.mkIf cfg.hyperkitty.enable {
         class = "mailman_hyperkitty.Archiver";
@@ -211,14 +225,22 @@ in {
               See <https://mailman.readthedocs.io/en/latest/src/mailman/docs/mta.html>.
             '';
           };
-    in [
+    in (lib.optionals cfg.enablePostfix [
       { assertion = postfix.enable;
-        message = "Mailman requires Postfix";
+        message = ''
+          Mailman's default NixOS configuration requires Postfix to be enabled.
+
+          If you want to use another MTA, set services.mailman.enablePostfix
+          to false and configure settings in services.mailman.settings.mta.
+
+          Refer to <https://mailman.readthedocs.io/en/latest/src/mailman/docs/mta.html>
+          for more info.
+        '';
       }
       (requirePostfixHash [ "relayDomains" ] "postfix_domains")
       (requirePostfixHash [ "config" "transport_maps" ] "postfix_lmtp")
       (requirePostfixHash [ "config" "local_recipient_maps" ] "postfix_lmtp")
-    ];
+    ]);
 
     users.users.mailman = {
       description = "GNU Mailman";
@@ -275,7 +297,7 @@ in {
       '';
     }) ];
 
-    services.postfix = {
+    services.postfix = lib.mkIf cfg.enablePostfix {
       recipientDelimiter = "+";         # bake recipient addresses in mail envelopes via VERP
       config = {
         owner_request_special = "no";   # Mailman handles -owner addresses on its own

nixos/modules/services/mail/mailman.xml

@@ -13,9 +13,9 @@
   </para>
 
   <section xml:id="module-services-mailman-basic-usage">
-    <title>Basic usage</title>
+    <title>Basic usage with Postfix</title>
     <para>
-      For a basic configuration, the following settings are suggested:
+      For a basic configuration with Postfix as the MTA, the following settings are suggested:
       <programlisting>{ config, ... }: {
   services.postfix = {
     enable = true;
@@ -56,4 +56,39 @@
       necessary, but outside the scope of the Mailman module.
     </para>
   </section>
+  <section xml:id="module-services-mailman-other-mtas">
+    <title>Using with other MTAs</title>
+    <para>
+      Mailman also supports other MTA, though with a little bit more configuration. For example, to use Mailman with Exim, you can use the following settings:
+      <programlisting>{ config, ... }: {
+  services = {
+    mailman = {
+      enable = true;
+      siteOwner = "mailman@example.org";
+      <link linkend="opt-services.mailman.enablePostfix">enablePostfix</link> = false;
+      settings.mta = {
+        incoming = "mailman.mta.exim4.LMTP";
+        outgoing = "mailman.mta.deliver.deliver";
+        lmtp_host = "localhost";
+        lmtp_port = "8024";
+        smtp_host = "localhost";
+        smtp_port = "25";
+        configuration = "python:mailman.config.exim4";
+      };
+    };
+    exim = {
+      enable = true;
+      # You can configure Exim in a separate file to reduce configuration.nix clutter
+      config = builtins.readFile ./exim.conf;
+    };
+  };
+}</programlisting>
+    </para>
+    <para>
+      The exim config needs some special additions to work with Mailman. Currently
+      NixOS can't manage Exim config with such granularity. Please refer to
+      <link xlink:href="https://mailman.readthedocs.io/en/latest/src/mailman/docs/mta.html">Mailman documentation</link>
+      for more info on configuring Mailman for working with Exim.
+    </para>
+  </section>
 </chapter>