Fix relay-without-auth MUA config (#4298)

config/runtime.exs

@@ -504,13 +504,24 @@ case mailer_adapter do
 
     if relay = get_var_from_path_or_env(config_dir, "SMTP_HOST_ADDR") do
       port = get_int_from_path_or_env(config_dir, "SMTP_HOST_PORT", 25)
+      config :plausible, Plausible.Mailer, relay: relay, port: port
+    end
+
     username = get_var_from_path_or_env(config_dir, "SMTP_USER_NAME")
     password = get_var_from_path_or_env(config_dir, "SMTP_USER_PWD")
 
-      config :plausible, Plausible.Mailer,
-        auth: [username: username, password: password],
-        relay: relay,
-        port: port
+    cond do
+      username && password ->
+        config :plausible, Plausible.Mailer, auth: [username: username, password: password]
+
+      username || password ->
+        raise ArgumentError, """
+        Both SMTP_USER_NAME and SMTP_USER_PWD must be set for SMTP authentication.
+        Please provide values for both environment variables.
+        """
+
+      _both_nil = true ->
+        nil
     end
 
   "Bamboo.LocalAdapter" ->

test/plausible/config_test.exs

@@ -159,9 +159,25 @@ defmodule Plausible.ConfigTest do
 
       assert get_in(runtime_config(env), [:plausible, Plausible.Mailer]) == [
                {:adapter, Bamboo.Mua},
-               {:auth, [username: "neo", password: "one"]},
                {:relay, "localhost"},
-               {:port, 2525}
+               {:port, 2525},
+               {:auth, [username: "neo", password: "one"]}
+             ]
+    end
+
+    test "Bamboo.Mua (no auth relay config)" do
+      env = [
+        {"MAILER_ADAPTER", "Bamboo.Mua"},
+        {"SMTP_HOST_ADDR", "localhost"},
+        {"SMTP_HOST_PORT", "2525"},
+        {"SMTP_USER_NAME", nil},
+        {"SMTP_USER_PWD", nil}
+      ]
+
+      assert get_in(runtime_config(env), [:plausible, Plausible.Mailer]) == [
+               adapter: Bamboo.Mua,
+               relay: "localhost",
+               port: 2525
              ]
     end