Remove unused multipart/mixed when there's no attachments

lib/bamboo/adapters/smtp_adapter.ex

@@ -276,6 +276,24 @@ defmodule Bamboo.SMTPAdapter do
     |> add_smtp_line("X-Attachment-Id: #{random}")
   end
 
+  defp add_multipart_attachment_headers(
+         body,
+         %Bamboo.Email{attachments: []},
+         _multi_part_mixed_delimiter
+       ),
+       do: body
+
+  defp add_multipart_attachment_headers(
+         body,
+         %Bamboo.Email{attachments: _attachments},
+         multi_part_mixed_delimiter
+       ) do
+    body
+    |> add_multipart_mixed_header(multi_part_mixed_delimiter)
+    |> add_ending_header
+    |> add_multipart_delimiter(multi_part_mixed_delimiter)
+  end
+
   defp add_attachment_body(body, data) do
     data =
       if String.contains?(body, "Content-Type: message/rfc822") do
@@ -297,14 +315,14 @@ defmodule Bamboo.SMTPAdapter do
     |> add_attachment_body(attachment.data)
   end
 
-  defp add_attachments(body, %Bamboo.Email{attachments: nil}, _), do: body
+  defp add_attachments(body, %Bamboo.Email{attachments: []}, _), do: body
 
   defp add_attachments(body, %Bamboo.Email{attachments: attachments}, multi_part_mixed_delimiter) do
     attachment_part =
       attachments
       |> Enum.map(fn attachment -> add_attachment(attachment, multi_part_mixed_delimiter) end)
 
-    "#{body}#{attachment_part}"
+    add_ending_multipart("#{body}#{attachment_part}", multi_part_mixed_delimiter)
   end
 
   defp add_to(body, %Bamboo.Email{to: recipients}) do
@@ -342,16 +360,13 @@ defmodule Bamboo.SMTPAdapter do
     |> add_to(email)
     |> add_custom_headers(email)
     |> add_mime_header
-    |> add_multipart_mixed_header(multi_part_mixed_delimiter)
-    |> add_ending_header
-    |> add_multipart_delimiter(multi_part_mixed_delimiter)
+    |> add_multipart_attachment_headers(email, multi_part_mixed_delimiter)
     |> add_multipart_header(multi_part_delimiter)
     |> add_ending_header
     |> add_text_body(email, multi_part_delimiter)
     |> add_html_body(email, multi_part_delimiter)
     |> add_ending_multipart(multi_part_delimiter)
     |> add_attachments(email, multi_part_mixed_delimiter)
-    |> add_ending_multipart(multi_part_mixed_delimiter)
   end
 
   defp build_error({:ok, value}, _key, errors) when value != nil, do: errors

test/lib/bamboo/adapters/smtp_adapter_test.exs

@@ -421,7 +421,7 @@ defmodule Bamboo.SMTPAdapterTest do
       SMTPAdapter.deliver(bamboo_email, bamboo_config)
   end
 
-  test "emails looks fine when only text body is set" do
+  test "emails looks fine when only html body is set" do
     bamboo_email = new_email(text_body: nil)
     bamboo_config = configuration()
 
@@ -579,6 +579,37 @@ defmodule Bamboo.SMTPAdapterTest do
     assert_configuration(bamboo_config, gen_smtp_config)
   end
 
+  test "emails contain multipart/mixed only when sent with attachments" do
+    bamboo_email = new_email()
+    bamboo_config = configuration()
+
+    {:ok, "200 Ok 1234567890"} = SMTPAdapter.deliver(bamboo_email, bamboo_config)
+
+    bamboo_email_with_attachments =
+      new_email()
+      |> Bamboo.Email.put_attachment(Path.absname("test/attachments/attachment_two.txt"),
+        filename: "some_attachment.txt"
+      )
+
+    {:ok, "200 Ok 1234567890"} = SMTPAdapter.deliver(bamboo_email_with_attachments, bamboo_config)
+
+    assert 2 = length(FakeGenSMTP.fetch_sent_emails())
+
+    [{{_, _, raw_email_2}, _}, {{_, _, raw_email}, _}] = FakeGenSMTP.fetch_sent_emails()
+
+    assert String.contains?(raw_email, "MIME-Version: 1.0\r\n")
+
+    refute String.contains?(
+             raw_email,
+             "multipart/mixed"
+           )
+
+    assert String.contains?(
+             raw_email_2,
+             "multipart/mixed"
+           )
+  end
+
   test "email looks fine when no bcc: is set" do
     bamboo_email = new_email(bcc: [])
     bamboo_config = configuration()