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.

1 files changed, 37 insertions, 2 deletions

diff --git a/nixos/modules/services/mail/mailman.xml b/nixos/modules/services/mail/mailman.xml
index cbe50ed0b91..8da491ccbe9 100644
--- a/nixos/modules/services/mail/mailman.xml
+++ b/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>