Only encode headers with non-ASCII characters

lib/mail/renderers/rfc_2822.ex

@@ -207,13 +207,22 @@ defmodule Mail.Renderers.RFC2822 do
   # As stated at https://datatracker.ietf.org/doc/html/rfc2047#section-2, encoded words must be
   # split in 76 chars including its surroundings and delimmiters.
   # Since enclosing starts with =?UTF-8?Q? and ends with ?=, max length should be 64
+  # Per RFC 2047, encoding is only required for non-ASCII characters.
+  # ASCII-only headers should not be encoded, regardless of length.
+  # Per RFC 2047, ASCII-only headers should not be encoded, regardless of length
   defp encode_header_value(header_value, :quoted_printable) do
-    case Mail.Encoders.QuotedPrintable.encode(header_value, 64) do
-      ^header_value -> header_value
-      encoded -> wrap_encoded_words(encoded)
+    if contains_non_ascii?(header_value) do
+      header_value |> Mail.Encoders.QuotedPrintable.encode(64) |> wrap_encoded_words()
+    else
+      header_value
     end
   end
 
+  # Check if a string contains any non-ASCII characters (bytes > 0x7F)
+  defp contains_non_ascii?(<<>>), do: false
+  defp contains_non_ascii?(<<byte, _rest::binary>>) when byte > 127, do: true
+  defp contains_non_ascii?(<<_byte, rest::binary>>), do: contains_non_ascii?(rest)
+
   defp wrap_encoded_words(value) do
     :binary.split(value, "=\r\n", [:global])
     |> Enum.map(fn chunk -> <<"=?UTF-8?Q?", chunk::binary, "?=">> end)

test/mail/renderers/rfc_2822_test.exs

@@ -41,6 +41,28 @@ defmodule Mail.Renderers.RFC2822Test do
     assert header == "Content-Disposition: attachment; filename=\"my;test;file\""
   end
 
+  test "encodes header if necessary" do
+    assert Mail.Renderers.RFC2822.render_header("Subject", [
+             "Hello World!"
+           ]) == "Subject: Hello World!"
+
+    assert Mail.Renderers.RFC2822.render_header("Subject", [
+             String.duplicate("a", 73)
+           ]) == "Subject: #{String.duplicate("a", 73)}"
+
+    assert Mail.Renderers.RFC2822.render_header("Subject", [
+             "Hello World 😀"
+           ]) == "Subject: =?UTF-8?Q?Hello World =F0=9F=98=80?="
+
+    assert Mail.Renderers.RFC2822.render_header("Subject", [
+             "Café résumé"
+           ]) == "Subject: =?UTF-8?Q?Caf=C3=A9 r=C3=A9sum=C3=A9?="
+
+    assert Mail.Renderers.RFC2822.render_header("Subject", [
+             "Hello 世界 World"
+           ]) == "Subject: =?UTF-8?Q?Hello =E4=B8=96=E7=95=8C World?="
+  end
+
   test "address headers renders list of recipients" do
     header = Mail.Renderers.RFC2822.render_header("from", "user1@example.com")
     assert header == "From: user1@example.com"