feat: per-message extra headers in the mail envelope

mail/sender.go

@@ -20,6 +20,7 @@ import (
 	"net/smtp"
 	"net/textproto"
 	"os"
+	"sort"
 	"strings"
 	"time"
 )
@@ -76,6 +77,11 @@ type message struct {
 	Subject string
 	HTML    string
 	Text    string
+
+	// Headers are extra header lines for this one message, such as an
+	// announcement's List-Unsubscribe pair. Keys and values are CR/LF-stripped
+	// like every other header.
+	Headers map[string]string
 }
 
 // sendTemplate renders name against data and delivers the result.
@@ -226,6 +232,14 @@ func (s Sender) envelope(m message) ([]byte, error) {
 	b.WriteString("Message-ID: " + messageID(s.From) + "\r\n")
 	b.WriteString("Subject: " + mime.QEncoding.Encode("utf-8", sanitizeHeader(m.Subject)) + "\r\n")
 	b.WriteString("MIME-Version: 1.0\r\n")
+	keys := make([]string, 0, len(m.Headers))
+	for k := range m.Headers {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	for _, k := range keys {
+		b.WriteString(sanitizeHeader(k) + ": " + sanitizeHeader(m.Headers[k]) + "\r\n")
+	}
 	b.WriteString("Content-Type: multipart/alternative; boundary=" + w.Boundary() + "\r\n")
 	b.WriteString("\r\n")
 	b.WriteString(parts.String())