#13: implement Outbound Queue, Retry Backoff Engine & DSN Bounce Generator (RFC 3464)

services/protocols/internal/domain/valueobject/dsn_bounce.go

@@ -0,0 +1,64 @@
+package valueobject
+
+import (
+	"fmt"
+	"strings"
+	"time"
+)
+
+type DsnAction string
+
+const (
+	DsnActionFailed  DsnAction = "failed"
+	DsnActionDelayed DsnAction = "delayed"
+)
+
+// BuildDsnReport constructs an RFC 3464 compliant delivery status notification report payload.
+// Returns (payload, isLoopPrevented). If envelopeFrom is empty or "<>", isLoopPrevented is true and payload is nil.
+func BuildDsnReport(action DsnAction, envelopeFrom string, envelopeTo string, messageID string, reason string) ([]byte, bool) {
+	cleanFrom := strings.Trim(strings.TrimSpace(envelopeFrom), "<>")
+	if cleanFrom == "" {
+		return nil, true // Loop prevention per RFC 5321 section 4.5.5
+	}
+
+	boundary := fmt.Sprintf("=_Boundary_DSN_%d", time.Now().UnixNano())
+	nowStr := time.Now().Format(time.RFC1123Z)
+
+	status := "5.0.0"
+	subject := "Undeliverable Mail Notification"
+	if action == DsnActionDelayed {
+		status = "4.4.1"
+		subject = "Delivery Delay Notification"
+	}
+
+	var sb strings.Builder
+	sb.WriteString(fmt.Sprintf("From: Mail Delivery Subsystem <postmaster@lambdamail.local>\r\n"))
+	sb.WriteString(fmt.Sprintf("To: <%s>\r\n", cleanFrom))
+	sb.WriteString(fmt.Sprintf("Subject: %s\r\n", subject))
+	sb.WriteString(fmt.Sprintf("Date: %s\r\n", nowStr))
+	sb.WriteString(fmt.Sprintf("Content-Type: multipart/report; report-type=delivery-status; boundary=\"%s\"\r\n\r\n", boundary))
+
+	// Part 1: Human-readable narrative
+	sb.WriteString(fmt.Sprintf("--%s\r\n", boundary))
+	sb.WriteString("Content-Type: text/plain; charset=utf-8\r\n\r\n")
+	if action == DsnActionFailed {
+		sb.WriteString(fmt.Sprintf("This is an automated message from LambdaMail.\r\nYour message to <%s> could not be delivered.\r\nReason: %s\r\n\r\n", envelopeTo, reason))
+	} else {
+		sb.WriteString(fmt.Sprintf("This is an automated warning message from LambdaMail.\r\nDelivery to <%s> has been delayed.\r\nLambdaMail will continue attempting delivery.\r\nReason: %s\r\n\r\n", envelopeTo, reason))
+	}
+
+	// Part 2: Machine-readable delivery-status
+	sb.WriteString(fmt.Sprintf("--%s\r\n", boundary))
+	sb.WriteString("Content-Type: message/delivery-status\r\n\r\n")
+	sb.WriteString("Reporting-MTA: dns; mail.lambdamail.local\r\n")
+	sb.WriteString(fmt.Sprintf("Arrival-Date: %s\r\n\r\n", nowStr))
+
+	sb.WriteString(fmt.Sprintf("Final-Recipient: rfc822; %s\r\n", envelopeTo))
+	sb.WriteString(fmt.Sprintf("Action: %s\r\n", action))
+	sb.WriteString(fmt.Sprintf("Status: %s\r\n", status))
+	sb.WriteString(fmt.Sprintf("Diagnostic-Code: smtp; %s\r\n\r\n", reason))
+
+	sb.WriteString(fmt.Sprintf("--%s--\r\n", boundary))
+
+	return []byte(sb.String()), false
+}

services/protocols/internal/domain/valueobject/dsn_bounce_test.go

@@ -0,0 +1,36 @@
+package valueobject
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestBuildDsnReport_PreventsBounceLoopForEmptySender(t *testing.T) {
+	_, isLoop := BuildDsnReport(DsnActionFailed, "", "user@external.test", "msg123", "550 User unknown")
+	if !isLoop {
+		t.Errorf("expected loop prevention for empty sender")
+	}
+
+	_, isLoop = BuildDsnReport(DsnActionFailed, "<>", "user@external.test", "msg123", "550 User unknown")
+	if !isLoop {
+		t.Errorf("expected loop prevention for <> sender")
+	}
+}
+
+func TestBuildDsnReport_GeneratesValidMIME(t *testing.T) {
+	payload, isLoop := BuildDsnReport(DsnActionFailed, "sender@local.test", "recipient@remote.test", "msg123", "550 User unknown")
+	if isLoop || payload == nil {
+		t.Fatalf("unexpected loop signal for valid sender")
+	}
+
+	raw := string(payload)
+	if !strings.Contains(raw, "Content-Type: multipart/report; report-type=delivery-status") {
+		t.Errorf("missing multipart/report header: %s", raw)
+	}
+	if !strings.Contains(raw, "Action: failed") {
+		t.Errorf("missing Action: failed: %s", raw)
+	}
+	if !strings.Contains(raw, "Diagnostic-Code: smtp; 550 User unknown") {
+		t.Errorf("missing Diagnostic-Code: %s", raw)
+	}
+}