Distinguish between permanent and transient errors

This patch makes the queue and couriers distinguish between permanent and
transient errors when delivering mail to individual recipients.

Pipe delivery errors are always permanent.

Procmail delivery errors are almost always permanent, except if the command
exited with code 75, which is an indication of transient.

SMTP delivery errors are almost always transient, except if the DNS resolution
for the domain failed.

internal/courier/smtp.go

@@ -35,14 +35,19 @@ var (
 type SMTP struct {
 }
 
-func (s *SMTP) Deliver(from string, to string, data []byte) error {
+func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
 	tr := trace.New("goingSMTP", "Deliver")
 	defer tr.Finish()
 	tr.LazyPrintf("%s  ->  %s", from, to)
 
+	// TODO: Fall back to A if MX is not available.
 	mx, err := lookupMX(envelope.DomainOf(to))
 	if err != nil {
-		return tr.Errorf("Could not find mail server: %v", err)
+		// Note this is considered a permanent error.
+		// This is in line with what other servers (Exim) do. However, the
+		// downside is that temporary DNS issues can affect delivery, so we
+		// have to make sure we try hard enough on the lookup above.
+		return tr.Errorf("Could not find mail server: %v", err), true
 	}
 	tr.LazyPrintf("MX: %s", mx)
 
@@ -53,13 +58,13 @@ func (s *SMTP) Deliver(from string, to string, data []byte) error {
 retry:
 	conn, err := net.DialTimeout("tcp", mx+":"+*smtpPort, smtpDialTimeout)
 	if err != nil {
-		return tr.Errorf("Could not dial: %v", err)
+		return tr.Errorf("Could not dial: %v", err), false
 	}
 	conn.SetDeadline(time.Now().Add(smtpTotalTimeout))
 
 	c, err := smtp.NewClient(conn, mx)
 	if err != nil {
-		return tr.Errorf("Error creating client: %v", err)
+		return tr.Errorf("Error creating client: %v", err), false
 	}
 
 	// TODO: Keep track of hosts and MXs that we've successfully done TLS
@@ -74,7 +79,7 @@ retry:
 			// Unfortunately, many servers use self-signed certs, so if we
 			// fail verification we just try again without validating.
 			if insecure {
-				return tr.Errorf("TLS error: %v", err)
+				return tr.Errorf("TLS error: %v", err), false
 			}
 
 			insecure = true
@@ -91,31 +96,35 @@ retry:
 		tr.LazyPrintf("Insecure - not using TLS")
 	}
 
+	// TODO: check if the errors we get back are transient or not.
+	// Go's smtp does not allow us to do this, so leave for when we do it
+	// ourselves.
+
 	if err = c.Mail(from); err != nil {
-		return tr.Errorf("MAIL %v", err)
+		return tr.Errorf("MAIL %v", err), false
 	}
 
 	if err = c.Rcpt(to); err != nil {
-		return tr.Errorf("RCPT TO %v", err)
+		return tr.Errorf("RCPT TO %v", err), false
 	}
 
 	w, err := c.Data()
 	if err != nil {
-		return tr.Errorf("DATA %v", err)
+		return tr.Errorf("DATA %v", err), false
 	}
 	_, err = w.Write(data)
 	if err != nil {
-		return tr.Errorf("DATA writing: %v", err)
+		return tr.Errorf("DATA writing: %v", err), false
 	}
 
 	err = w.Close()
 	if err != nil {
-		return tr.Errorf("DATA closing %v", err)
+		return tr.Errorf("DATA closing %v", err), false
 	}
 
 	c.Quit()
 
-	return nil
+	return nil, false
 }
 
 func lookupMX(domain string) (string, error) {