queue: Send DSN for messages that time out in the queue

The queue currently only considers failed recipients when deciding
whether to send a DSN or not. This is a bug, as recipients that time out
are not taken into account.

This patch fixes that issue by including both failed and pending
recipients in the DSN.

It also adds more comprehensive tests for this case, both in the queue
and in the dsn generation code.

internal/queue/dsn.go

@@ -23,11 +23,18 @@ func deliveryStatusNotification(domainFrom string, item *Item) ([]byte, error) {
 		Date:        time.Now().Format(time.RFC1123Z),
 		To:          item.To,
 		Recipients:  item.Rcpt,
+		FailedTo:    map[string]string{},
 	}
 
 	for _, rcpt := range item.Rcpt {
-		if rcpt.Status == Recipient_FAILED {
-			info.FailedRcpts = append(info.FailedRcpts, rcpt)
+		if rcpt.Status != Recipient_SENT {
+			info.FailedTo[rcpt.OriginalAddress] = rcpt.OriginalAddress
+			switch rcpt.Status {
+			case Recipient_FAILED:
+				info.FailedRecipients = append(info.FailedRecipients, rcpt)
+			case Recipient_PENDING:
+				info.PendingRecipients = append(info.PendingRecipients, rcpt)
+			}
 		}
 	}
 
@@ -43,14 +50,16 @@ func deliveryStatusNotification(domainFrom string, item *Item) ([]byte, error) {
 }
 
 type dsnInfo struct {
-	OurDomain       string
-	Destination     string
-	MessageID       string
-	Date            string
-	To              []string
-	Recipients      []*Recipient
-	FailedRcpts     []*Recipient
-	OriginalMessage string
+	OurDomain         string
+	Destination       string
+	MessageID         string
+	Date              string
+	To                []string
+	FailedTo          map[string]string
+	Recipients        []*Recipient
+	FailedRecipients  []*Recipient
+	PendingRecipients []*Recipient
+	OriginalMessage   string
 }
 
 var dsnTemplate = template.Must(template.New("dsn").Parse(
@@ -59,17 +68,22 @@ To: <{{.Destination}}>
 Subject: Mail delivery failed: returning message to sender
 Message-ID: <{{.MessageID}}>
 Date: {{.Date}}
-X-Failed-Recipients: {{range .To}}{{.}}, {{end}}
+X-Failed-Recipients: {{range .FailedTo}}{{.}}, {{end}}
 Auto-Submitted: auto-replied
 
 Delivery to the following recipient(s) failed permanently:
 
-  {{range .To -}} - {{.}}
-  {{end}}
+  {{range .FailedTo -}} - {{.}}
+  {{- end}}
+
 
 ----- Technical details -----
-{{range .Recipients}}
-- "{{.Address}}" ({{.Type}}) failed with error:
+{{range .FailedRecipients}}
+- "{{.Address}}" ({{.Type}}) failed permanently with error:
+    {{.LastFailureMessage}}
+{{end}}
+{{- range .PendingRecipients}}
+- "{{.Address}}" ({{.Type}}) failed repeatedly and timed out, last error:
     {{.LastFailureMessage}}
 {{end}}