fix(mail): add Message-ID to email messages

server/internal/mail/mail.go

@@ -1,10 +1,13 @@
 package mail
 
 import (
+	"crypto/rand"
+	"encoding/hex"
 	"fmt"
 	"log"
 	"net/smtp"
 	"strings"
+	"time"
 
 	"wedding-server/internal/config"
 	"wedding-server/internal/store"
@@ -18,6 +21,19 @@ func New(cfg config.EmailConfig) *Mailer {
 	return &Mailer{cfg: cfg}
 }
 
+// newMessageID builds an RFC 5322 Message-ID, scoped to the sender's own
+// domain so it satisfies mail servers that reject messages lacking one.
+func newMessageID(from string) string {
+	domain := "localhost"
+	if i := strings.LastIndex(from, "@"); i != -1 {
+		domain = strings.TrimSuffix(from[i+1:], ">")
+	}
+
+	var buf [16]byte
+	_, _ = rand.Read(buf[:])
+	return fmt.Sprintf("<%d.%s@%s>", time.Now().UnixNano(), hex.EncodeToString(buf[:]), domain)
+}
+
 func (m *Mailer) send(subject, body string) error {
 	if m.cfg.SMTPHost == "" {
 		log.Printf("mail: SMTP not configured, skipping email %q", subject)
@@ -28,8 +44,8 @@ func (m *Mailer) send(subject, body string) error {
 	auth := smtp.PlainAuth("", m.cfg.SMTPUser, m.cfg.SMTPPass, m.cfg.SMTPHost)
 
 	msg := fmt.Sprintf(
-		"From: %s\r\nTo: %s\r\nSubject: %s\r\nContent-Type: text/plain; charset=UTF-8\r\n\r\n%s",
+		"From: %s\r\nTo: %s\r\nSubject: %s\r\nMessage-ID: %s\r\nDate: %s\r\nContent-Type: text/plain; charset=UTF-8\r\n\r\n%s",
-		m.cfg.From, m.cfg.To, subject, body,
+		m.cfg.From, m.cfg.To, subject, newMessageID(m.cfg.From), time.Now().Format(time.RFC1123Z), body,
 	)
 
 	return smtp.SendMail(addr, auth, m.cfg.From, []string{m.cfg.To}, []byte(msg))