upstream-tls-verify: Verified TLS to an inbound domain's upstream

Owner, 2026-10-18: "Оба предложения делаем"; the mockup is c10cd78. The
security audit's M7: Required is Postfix's encrypt level, which checks no
certificate - it keeps the mail from a listener, not from one who answers
in the upstream's place.

- A fourth TLS mode, Verified, stored as Postfix's level secure (migration
  0014 rebuilds inbound_transports with the wider CHECK; schema 14) and
  written to the TLS policy map like the others. Postfix matches the
  certificate against the next hop's name (smtp_tls_secure_cert_match =
  nexthop, dot-nexthop, the default; postconf(5)), so Verified is refused
  for an upstream given by address (checkSecureHost).
- Postfix's SMTP client trusts Debian's ca-certificates bundle
  (smtp_tls_CAfile): Postfix trusts none by default (DEF_SMTP_TLS_CA_FILE
  "", DEF_TLS_APPEND_DEF_CA 0). The levels may and encrypt only log the
  verdict, so mail to recipients' MX is unchanged.
- The page: the select's fourth option, a help line under it, the lead
  "over verified TLS", the list's tag "verified"; Help word for word with
  the mockup; guide § Inbound relay, with a posttls-finger command to try
  an upstream first.

Tests: TestVerifiedTLSNeedsTheUpstreamsName, TestRenderInboundMapsSecure,
TestUpstreamTLSModeSecure; schema tests at 14. e2e
inbound_upstream_verified_tls: an address refused, a name saved as
"[sink]:25 secure" in tls_policy, the page says verified, the CA file set
and present, and posttls-finger -l secure with it calls the stand's
self-signed port 25 "Untrusted" (the stand has no TLS upstream). make e2e
(65 passed) and go vet + go test (26 packages) on the test server for this
tree. Tried against the live upstream from the 2.14.5 container:
mc-ent.mixfed.ru gives "Verified TLS connection established".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/inbound/validate.go

@@ -103,13 +103,26 @@ func parsePort(raw string) (int, error) {
 
 func checkTLSMode(mode string) error {
 	switch mode {
-	case "may", "encrypt", "none":
+	case "may", "encrypt", "secure", "none":
 		return nil
 	default:
 		return invalid("invalid TLS mode")
 	}
 }
 
+// checkSecureHost refuses Verified TLS for an upstream given by address:
+// Postfix's secure level matches the certificate against the next hop's name
+// (smtp_tls_secure_cert_match = nexthop, dot-nexthop; Postfix 3.10.13,
+// postconf(5): "the transport next-hop configured for the domain stripped of
+// ... enclosing square brackets and trailing port"), and a certificate names
+// hosts, not addresses.
+func checkSecureHost(host, mode string) error {
+	if mode == "secure" && net.ParseIP(host) != nil {
+		return invalid("Verified TLS checks the upstream's certificate against its name: give the upstream by name, not by address")
+	}
+	return nil
+}
+
 func checkRecipientMode(mode string) error {
 	switch mode {
 	case "list", "any":