queue: Send delivery status notifications on failures

When we permanently failed to deliver to one or more recipients, send delivery
status notifications back to the sender.

To do this, we need to extend a couple of internal structures, to keep track
of the original destinations (so we can include them in the message, for
reference), and the hostname we're identifying ourselves as (this is arguable
but we're going with it for now, may change later).

internal/queue/dsn.go

@@ -0,0 +1,80 @@
+package queue
+
+import (
+	"bytes"
+	"text/template"
+	"time"
+)
+
+// Maximum length of the original message to include in the DSN.
+const maxOrigMsgLen = 256 * 1024
+
+// deliveryStatusNotification creates a delivery status notification (DSN) for
+// the given item, and puts it in the queue.
+//
+// There is a standard, https://tools.ietf.org/html/rfc3464, although most
+// MTAs seem to use a plain email and include an X-Failed-Recipients header.
+// We're going with the latter for now, may extend it to the former later.
+func deliveryStatusNotification(item *Item) ([]byte, error) {
+	info := dsnInfo{
+		OurDomain:   item.Hostname,
+		Destination: item.From,
+		MessageID:   "chasquid-dsn-" + <-newID + "@" + item.Hostname,
+		Date:        time.Now().Format(time.RFC1123Z),
+		To:          item.To,
+		Recipients:  item.Rcpt,
+	}
+
+	for _, rcpt := range item.Rcpt {
+		if rcpt.Status == Recipient_FAILED {
+			info.FailedRcpts = append(info.FailedRcpts, rcpt)
+		}
+	}
+
+	if len(item.Data) > maxOrigMsgLen {
+		info.OriginalMessage = string(item.Data[:maxOrigMsgLen])
+	} else {
+		info.OriginalMessage = string(item.Data)
+	}
+
+	buf := &bytes.Buffer{}
+	err := dsnTemplate.Execute(buf, info)
+	return buf.Bytes(), err
+}
+
+type dsnInfo struct {
+	OurDomain       string
+	Destination     string
+	MessageID       string
+	Date            string
+	To              []string
+	Recipients      []*Recipient
+	FailedRcpts     []*Recipient
+	OriginalMessage string
+}
+
+var dsnTemplate = template.Must(template.New("dsn").Parse(
+	`From: Mail Delivery System <postmaster-dsn@{{.OurDomain}}>
+To: <{{.Destination}}>
+Subject: Mail delivery failed: returning message to sender
+Message-ID: <{{.MessageID}}>
+Date: {{.Date}}
+X-Failed-Recipients: {{range .To}}{{.}}, {{end}}
+Auto-Submitted: auto-replied
+
+Delivery to the following recipient(s) failed permanently:
+
+  {{range .To -}} - {{.}}
+  {{end}}
+
+----- Technical details -----
+{{range .Recipients}}
+- "{{.Address}}" ({{.Type}}) failed with error:
+    {{.LastFailureMessage}}
+{{end}}
+
+----- Original message -----
+
+{{.OriginalMessage}}
+
+`))