Add optional inbound relay (backup-MX) behind INBOUND_RELAY_ENABLE.

Port 25 accepts only configured domains and listed recipients, then forwards to an upstream; the outbound path is unchanged when the flag is off.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/postfix/inbound.go

@@ -0,0 +1,130 @@
+package postfix
+
+import (
+	"fmt"
+	"net"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// InboundRoute is one inbound domain's Postfix map material: the relay
+// domain, the next-hop transport, the TLS policy for that hop, and the
+// recipient list (or a domain catch-all).
+type InboundRoute struct {
+	Domain        string
+	Host          string
+	Port          int
+	TLSMode       string
+	RecipientMode string
+	Recipients    []string
+}
+
+func (p *Postfix) inboundMapPaths() (relayDomains, transport, recipients, tlsPolicy string) {
+	dir := filepath.Dir(p.senderLoginMapsPath)
+	return filepath.Join(dir, "relay_domains"),
+		filepath.Join(dir, "transport"),
+		filepath.Join(dir, "relay_recipients"),
+		filepath.Join(dir, "tls_policy")
+}
+
+// RebuildInboundMaps regenerates the inbound relay lookup tables from the full
+// set of configured routes and reloads Postfix. Domains with an empty host are
+// omitted so mail is never accepted with nowhere to send it. Full regeneration
+// keeps the files a pure function of the registry (security.md).
+func (p *Postfix) RebuildInboundMaps(routes []InboundRoute) error {
+	relay, transport, recipients, tlsPolicy, err := renderInboundMaps(routes)
+	if err != nil {
+		return err
+	}
+	rd, tr, rc, tl := p.inboundMapPaths()
+	if err := writeFileAtomic(rd, relay, 0o640); err != nil {
+		return err
+	}
+	if err := writeFileAtomic(tr, transport, 0o640); err != nil {
+		return err
+	}
+	if err := writeFileAtomic(rc, recipients, 0o640); err != nil {
+		return err
+	}
+	if err := writeFileAtomic(tl, tlsPolicy, 0o640); err != nil {
+		return err
+	}
+	return p.reload()
+}
+
+func renderInboundMaps(routes []InboundRoute) (relay, transport, recipients, tlsPolicy []byte, err error) {
+	sort.Slice(routes, func(i, j int) bool { return routes[i].Domain < routes[j].Domain })
+
+	var relayB, transportB, recipB, tlsB strings.Builder
+	for _, r := range routes {
+		if strings.TrimSpace(r.Host) == "" {
+			continue
+		}
+		if err := assertInboundRouteSafe(r); err != nil {
+			return nil, nil, nil, nil, err
+		}
+		nexthop := inboundNexthop(r.Host, r.Port)
+		fmt.Fprintf(&relayB, "%s OK\n", r.Domain)
+		fmt.Fprintf(&transportB, "%s smtp:%s\n", r.Domain, nexthop)
+		fmt.Fprintf(&tlsB, "%s %s\n", nexthop, r.TLSMode)
+		switch r.RecipientMode {
+		case "any":
+			fmt.Fprintf(&recipB, "@%s OK\n", r.Domain)
+		default:
+			addrs := append([]string(nil), r.Recipients...)
+			sort.Strings(addrs)
+			for _, addr := range addrs {
+				fmt.Fprintf(&recipB, "%s OK\n", addr)
+			}
+		}
+	}
+	return []byte(relayB.String()), []byte(transportB.String()), []byte(recipB.String()), []byte(tlsB.String()), nil
+}
+
+// inboundNexthop is the Postfix next-hop [host]:port form that disables MX
+// lookup for the explicit upstream.
+func inboundNexthop(host string, port int) string {
+	if ip := net.ParseIP(host); ip != nil && ip.To4() == nil {
+		return "[" + host + "]:" + strconv.Itoa(port)
+	}
+	return "[" + host + "]:" + strconv.Itoa(port)
+}
+
+func assertInboundRouteSafe(r InboundRoute) error {
+	if err := assertMapToken(r.Domain, "domain"); err != nil {
+		return err
+	}
+	if err := assertMapToken(r.Host, "host"); err != nil {
+		return err
+	}
+	if r.Port < 1 || r.Port > 65535 {
+		return fmt.Errorf("postfix: invalid inbound port %d", r.Port)
+	}
+	switch r.TLSMode {
+	case "may", "encrypt", "none":
+	default:
+		return fmt.Errorf("postfix: invalid tls mode %q", r.TLSMode)
+	}
+	if r.RecipientMode != "list" && r.RecipientMode != "any" {
+		return fmt.Errorf("postfix: invalid recipient mode %q", r.RecipientMode)
+	}
+	for _, addr := range r.Recipients {
+		if err := assertMapToken(addr, "recipient"); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// assertMapToken rejects values that could break out of a texthash line.
+func assertMapToken(v, what string) error {
+	if v == "" {
+		return fmt.Errorf("postfix: empty %s", what)
+	}
+	if strings.ContainsAny(v, " \t\r\n,\\") {
+		return fmt.Errorf("postfix: unsafe character in %s %q", what, v)
+	}
+	return nil
+}