Merge branch 'fix/smtp-date-messageid-headers'

internal/otpdelivery/smtp/sender.go

@@ -3,7 +3,9 @@ package smtp
 import (
 	"bytes"
 	"context"
+	"crypto/rand"
 	"crypto/tls"
+	"encoding/hex"
 	"fmt"
 	"mime"
 	"net"
@@ -146,6 +148,8 @@ func buildMessage(from, to, subject, body string) []byte {
 	var b bytes.Buffer
 	b.WriteString("From: " + from + "\r\n")
 	b.WriteString("To: " + to + "\r\n")
+	b.WriteString("Date: " + time.Now().Format(time.RFC1123Z) + "\r\n")
+	b.WriteString("Message-ID: " + generateMessageID(from) + "\r\n")
 	b.WriteString("Subject: " + mime.QEncoding.Encode("utf-8", subject) + "\r\n")
 	b.WriteString("MIME-Version: 1.0\r\n")
 	b.WriteString("Content-Type: text/plain; charset=utf-8\r\n")
@@ -155,6 +159,25 @@ func buildMessage(from, to, subject, body string) []byte {
 	return b.Bytes()
 }
 
+// generateMessageID builds a Message-ID header value (RFC 5322 3.6.4), using
+// the sending domain parsed out of the From address and a random token so
+// each message gets a unique id even under concurrent sends.
+func generateMessageID(from string) string {
+	domain := "localhost"
+	if addr, err := stdmail.ParseAddress(from); err == nil {
+		if i := strings.LastIndex(addr.Address, "@"); i >= 0 {
+			domain = addr.Address[i+1:]
+		}
+	}
+	var raw [16]byte
+	if _, err := rand.Read(raw[:]); err != nil {
+		// crypto/rand failing is effectively unheard of, but fall back to a
+		// time-based token rather than emit a non-unique Message-ID.
+		return fmt.Sprintf("<%d@%s>", time.Now().UnixNano(), domain)
+	}
+	return fmt.Sprintf("<%s@%s>", hex.EncodeToString(raw[:]), domain)
+}
+
 // emailContent builds the login-code email subject/body, branded with the
 // operator's configured product name (Config.AppName) instead of the
 // package's internal "telesrv" fallback.