fix: send email parts as quoted-printable so they are 7-bit clean and DKIM-signed

mail/sender.go

@@ -14,6 +14,7 @@ import (
 	"fmt"
 	"mime"
 	"mime/multipart"
+	"mime/quotedprintable"
 	"net"
 	"net/smtp"
 	"net/textproto"
@@ -180,27 +181,21 @@ func recipients(to string) []string {
 // text part is sent beside every HTML one for the same reason, and because a
 // client that refuses HTML should not receive a blank message. Header values
 // are stripped of CR and LF so a crafted instance name cannot inject headers.
+//
+// Both parts are quoted-printable. They were raw UTF-8 with no
+// Content-Transfer-Encoding, which means 7bit, and every template carries
+// non-ASCII (the middot in the masthead at least). rspamd scored that
+// R_BAD_CTE_7BIT, the message went out without a DKIM signature, and Gmail
+// filed it as spam, while mail from the same mailbox via SOGo, sent
+// quoted-printable, was signed and delivered.
 func (s Sender) envelope(m message) ([]byte, error) {
 	var parts strings.Builder
 	w := multipart.NewWriter(&parts)
 
-	textPart, err := w.CreatePart(textproto.MIMEHeader{
-		"Content-Type": {"text/plain; charset=utf-8"},
-	})
-	if err != nil {
+	if err := writeQPPart(w, "text/plain; charset=utf-8", m.Text); err != nil {
 		return nil, err
 	}
-	if _, err := textPart.Write([]byte(m.Text)); err != nil {
-		return nil, err
-	}
-
-	htmlPart, err := w.CreatePart(textproto.MIMEHeader{
-		"Content-Type": {"text/html; charset=utf-8"},
-	})
-	if err != nil {
-		return nil, err
-	}
-	if _, err := htmlPart.Write([]byte(m.HTML)); err != nil {
+	if err := writeQPPart(w, "text/html; charset=utf-8", m.HTML); err != nil {
 		return nil, err
 	}
 
@@ -224,6 +219,23 @@ func (s Sender) envelope(m message) ([]byte, error) {
 	return []byte(b.String()), nil
 }
 
+// writeQPPart adds one quoted-printable part, so the wire stays 7-bit and
+// lines stay under SMTP's 998-byte limit whatever the template produced.
+func writeQPPart(w *multipart.Writer, contentType, body string) error {
+	part, err := w.CreatePart(textproto.MIMEHeader{
+		"Content-Type":              {contentType},
+		"Content-Transfer-Encoding": {"quoted-printable"},
+	})
+	if err != nil {
+		return err
+	}
+	qp := quotedprintable.NewWriter(part)
+	if _, err := qp.Write([]byte(body)); err != nil {
+		return err
+	}
+	return qp.Close()
+}
+
 func messageID(from string) string {
 	domain := "vantage.local"
 	if at := strings.LastIndex(from, "@"); at >= 0 && at < len(from)-1 {

mail/sender_test.go

@@ -0,0 +1,70 @@
+package mail
+
+import (
+	"bytes"
+	"io"
+	"mime"
+	"mime/multipart"
+	"mime/quotedprintable"
+	"net/mail"
+	"strings"
+	"testing"
+)
+
+// Every part must declare quoted-printable and put only 7-bit bytes on the
+// wire. Raw UTF-8 under an implied 7bit encoding scored R_BAD_CTE_7BIT in
+// rspamd, went out without a DKIM signature, and landed in Gmail's spam.
+func TestEnvelopePartsAreQuotedPrintable(t *testing.T) {
+	s := Sender{From: "Vantage <support@example.com>"}
+	m := message{
+		To:      "a@example.com",
+		Subject: "Smith & Sons · restored",
+		Text:    "VANTAGE · Account\nBilling has resumed.",
+		HTML:    "<p>VANTAGE · Account</p><p>" + strings.Repeat("x", 2000) + "</p>",
+	}
+	raw, err := s.envelope(m)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, c := range raw {
+		if c > 0x7e && c != '\r' && c != '\n' {
+			t.Fatalf("byte %d is 0x%x: the wire must be 7-bit", i, c)
+		}
+	}
+	for _, line := range strings.Split(string(raw), "\r\n") {
+		if len(line) > 998 {
+			t.Fatalf("line of %d bytes exceeds SMTP's 998", len(line))
+		}
+	}
+
+	msg, err := mail.ReadMessage(bytes.NewReader(raw))
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, params, err := mime.ParseMediaType(msg.Header.Get("Content-Type"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r := multipart.NewReader(msg.Body, params["boundary"])
+	want := []string{m.Text, m.HTML}
+	for i := range want {
+		p, err := r.NextRawPart()
+		if err != nil {
+			t.Fatalf("part %d: %v", i, err)
+		}
+		if cte := p.Header.Get("Content-Transfer-Encoding"); cte != "quoted-printable" {
+			t.Fatalf("part %d Content-Transfer-Encoding = %q", i, cte)
+		}
+		// NextPart would decode for us; NextRawPart keeps the check honest by
+		// decoding here, so a missing header cannot pass by accident.
+		body, err := io.ReadAll(quotedprintable.NewReader(p))
+		if err != nil {
+			t.Fatal(err)
+		}
+		// Quoted-printable text mode writes line breaks as CRLF, which is the
+		// canonical form on the wire.
+		if strings.ReplaceAll(string(body), "\r\n", "\n") != want[i] {
+			t.Fatalf("part %d decodes to %q", i, body)
+		}
+	}
+}