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/web/validate/validate_test.go

@@ -65,3 +65,69 @@ func TestValidateDomainLongLabelRejected(t *testing.T) {
 		t.Error("expected error for over-long label")
 	}
 }
+
+func TestHostValid(t *testing.T) {
+	valid := []string{
+		"10.0.0.8",
+		"192.0.2.20",
+		"2001:db8::1",
+		"mail.internal.example",
+		"mx1",
+		"mail-1.lan",
+	}
+	for _, h := range valid {
+		if err := Host(h); err != nil {
+			t.Errorf("Host(%q) unexpected error: %v", h, err)
+		}
+	}
+}
+
+func TestHostInvalid(t *testing.T) {
+	invalid := []string{
+		"",
+		"exa mple",
+		"host/name",
+		"host;rm",
+		"-bad",
+		"bad-",
+		"host\nname",
+	}
+	for _, h := range invalid {
+		if err := Host(h); err == nil {
+			t.Errorf("Host(%q) = nil, want error", h)
+		}
+	}
+}
+
+func TestNormalizeHostStripsIPv6Brackets(t *testing.T) {
+	if got := NormalizeHost("  [2001:DB8::1] "); got != "2001:db8::1" {
+		t.Errorf("NormalizeHost IPv6 = %q", got)
+	}
+	if got := NormalizeHost("Mail.Example.COM"); got != "mail.example.com" {
+		t.Errorf("NormalizeHost hostname = %q", got)
+	}
+}
+
+func TestPort(t *testing.T) {
+	n, err := Port("25")
+	if err != nil || n != 25 {
+		t.Fatalf("Port(25) = %d, %v", n, err)
+	}
+	for _, raw := range []string{"", "0", "65536", "abc", "-1"} {
+		if _, err := Port(raw); err == nil {
+			t.Errorf("Port(%q) = nil, want error", raw)
+		}
+	}
+}
+
+func TestMailboxInDomain(t *testing.T) {
+	if err := MailboxInDomain("staff@lists.example.com", "lists.example.com"); err != nil {
+		t.Fatal(err)
+	}
+	if err := MailboxInDomain("staff@other.com", "lists.example.com"); err == nil {
+		t.Fatal("expected domain mismatch error")
+	}
+	if err := MailboxInDomain("bad addr@lists.example.com", "lists.example.com"); err == nil {
+		t.Fatal("expected local-part error")
+	}
+}