A multipart message with a single part will respect the multipart structure

lib/mail/renderers/rfc_2822.ex

@@ -300,14 +300,12 @@ defmodule Mail.Renderers.RFC2822 do
         Mail.Message.put_content_type(message, "multipart/mixed")
       else
         alternative =
-          case text_parts do
-            [part] ->
-              part
-
-            text_parts ->
-              Mail.build_multipart()
-              |> Mail.Message.put_content_type("multipart/alternative")
-              |> Mail.Message.put_parts(text_parts)
+          if match?([_part], text_parts) && attachments != [] do
+            List.first(text_parts)
+          else
+            Mail.build_multipart()
+            |> Mail.Message.put_content_type("multipart/alternative")
+            |> Mail.Message.put_parts(text_parts)
           end
 
         related =

test/mail/renderers/rfc_2822_test.exs

@@ -410,6 +410,30 @@ defmodule Mail.Renderers.RFC2822Test do
   end
 
   describe "multipart configuration" do
+    # This test ensures that Mail.build_multipart() is respected even if only one part is supplied
+    test "multipart/alternative with only one part and no attachments" do
+      message =
+        Mail.build_multipart()
+        |> Mail.put_to("user1@example.com")
+        |> Mail.put_from({"User2", "user2@example.com"})
+        |> Mail.put_subject("Test email")
+        |> Mail.put_text("Some text")
+        |> Mail.Renderers.RFC2822.render()
+        |> Mail.Parsers.RFC2822.parse()
+
+      assert %Mail.Message{
+               headers: %{"content-type" => ["multipart/alternative", {"boundary", _boundary}]},
+               parts: [
+                 %Mail.Message{
+                   headers: %{"content-type" => ["text/plain", {"charset", "UTF-8"}]},
+                   body: "Some text",
+                   parts: [],
+                   multipart: false
+                 }
+               ]
+             } = message
+    end
+
     test "multipart/alternative with text/plain and text/html" do
       message =
         Mail.build_multipart()