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 {