aliases: Implement "via" aliases

This patch implements "via" aliases, which let us explicitly select a
server to use for delivery.

This feature is useful in different scenarios, such as a secondary MX
server that forwards all incoming email to a primary.

For now, it is experimental and the syntax and semantics are subject to
change.

internal/courier/courier.go

@@ -8,4 +8,9 @@ type Courier interface {
 	// Deliver mail to a recipient. Return the error (if any), and whether it
 	// is permanent (true) or transient (false).
 	Deliver(from string, to string, data []byte) (error, bool)
+
+	// Forward mail using the given servers.
+	// Return the error (if any), and whether it is permanent (true) or
+	// transient (false).
+	Forward(from string, to string, data []byte, servers []string) (error, bool)
 }

internal/courier/mda.go

@@ -3,6 +3,7 @@ package courier
 import (
 	"bytes"
 	"context"
+	"errors"
 	"fmt"
 	"os/exec"
 	"strings"
@@ -108,3 +109,11 @@ func sanitizeForMDA(s string) string {
 	}
 	return strings.Map(valid, s)
 }
+
+var errForwardNotSupported = errors.New(
+	"forwarding not supported by the MDA courier")
+
+// Forward is not supported by the MDA courier.
+func (p *MDA) Forward(from string, to string, data []byte, servers []string) (error, bool) {
+	return errForwardNotSupported, true
+}

internal/courier/mda_test.go

@@ -123,3 +123,15 @@ func TestSanitize(t *testing.T) {
 		}
 	}
 }
+
+func TestForward(t *testing.T) {
+	p := MDA{"thisdoesnotexist", nil, 1 * time.Minute}
+	err, permanent := p.Forward(
+		"from", "to", []byte("data"), []string{"server"})
+	if err != errForwardNotSupported {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if !permanent {
+		t.Errorf("expected permanent, got transient")
+	}
+}

internal/courier/smtp.go

@@ -63,7 +63,7 @@ func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
 		to:       to,
 		toDomain: envelope.DomainOf(to),
 		data:     data,
-		tr:       trace.New("Courier.SMTP", to),
+		tr:       trace.New("Courier.SMTP.Deliver", to),
 	}
 	defer a.tr.Finish()
 	a.tr.Debugf("%s  ->  %s", from, to)
@@ -105,6 +105,42 @@ func (s *SMTP) Deliver(from string, to string, data []byte) (error, bool) {
 	return a.tr.Errorf("all MXs returned transient failures (last: %v)", err), false
 }
 
+// Forward an email. On failures, returns an error, and whether or not it is
+// permanent.
+func (s *SMTP) Forward(from string, to string, data []byte, servers []string) (error, bool) {
+	a := &attempt{
+		courier:  s,
+		from:     from,
+		to:       to,
+		toDomain: envelope.DomainOf(to),
+		data:     data,
+		tr:       trace.New("Courier.SMTP.Forward", to),
+	}
+	defer a.tr.Finish()
+	a.tr.Debugf("%s  ->  %s", from, to)
+
+	// smtp.Client.Mail will add the <> for us when the address is empty.
+	if a.from == "<>" {
+		a.from = ""
+	}
+
+	var err error
+	for _, server := range servers {
+		var permanent bool
+		err, permanent = a.deliver(server)
+		if err == nil {
+			return nil, false
+		}
+		if permanent {
+			return err, true
+		}
+		a.tr.Errorf("%q returned transient error: %v", server, err)
+	}
+
+	// We exhausted all servers, try again later.
+	return a.tr.Errorf("all servers returned transient failures (last: %v)", err), false
+}
+
 type attempt struct {
 	courier *SMTP