e2e: a negative case passes only on its own refusal (audit stage 0)

sendResult.refusedAt(stage, code, text) and replyIs(err, code, text):
nil only for the server's reply with that code and text at that stage;
a reset, a timeout or another reply is described for the failure.

- level 1: only anvil's "450 4.7.1 Error: too much mail from", only after
  a message of the loop was accepted. Held to that, it failed twice on
  the stand, each time for a real reason:
  1. waitForSMTPSReady took a TCP connect for readiness; Docker's port
     proxy accepts before Postfix listens and then resets - the reset
     once taken for the limit. It now waits for the TLS 220 greeting.
  2. the first message then got "553 ... not owned by user l1app": an
     smtpd idle from before the panel's postfix reload took the
     connection with the old sender_login_maps. The level-1 limit had
     never been what the case saw. The first message now waits (15 s at
     most) for the map, failing at once on any other refusal.
  Now: "level-1 limit tripped on attempt 51/60: 450 4.7.1 Error: too
  much mail from 10.77.0.1".
- level 2: 451 4.7.1 at MAIL, and both MAIL FROM of one connection.
- sender mismatch: 553 at RCPT, "not owned by user".
- no AUTH and the relay case: 554 "Client host rejected: Access
  denied" at RCPT; the relay case's comment claimed
  reject_unauth_destination, which on 465 comes after the client
  restriction - port 25 holds "554 Relay access denied".
- port 25: 550 5.1.1 for unknown recipients and report addresses.
- panel down: a 4xx reply (textproto), not just text starting with 4.

The product side (a second after an application is saved when its first
message can get 553) is written in the audit plan, not changed. Audit plan
line ticked with what was found, roadmap 7/61. No CHANGELOG line: tests only.

Evidence: make e2e on the test server for this tree, exit 0.

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

test/e2e/sender_check_test.go

@@ -1,7 +1,9 @@
 package e2e
 
 import (
+	"errors"
 	"fmt"
+	"net/textproto"
 	"regexp"
 	"strconv"
 	"strings"
@@ -127,8 +129,9 @@ func testPanelDownDefersMail(t *testing.T, sc *scenario) {
 	// Temporary, and before any message is offered: Postfix answers 451 from
 	// the connection on, and "454 4.3.0 Try again later" at AUTH, which is
 	// where a client that authenticates gives up.
-	if err := res.firstErr(); err == nil || !strings.HasPrefix(fmt.Sprint(err), "4") {
-		t.Fatalf("want a temporary 4xx with the panel down, got %v", res.firstErr())
+	var reply *textproto.Error
+	if err := res.firstErr(); !errors.As(err, &reply) || reply.Code/100 != 4 {
+		t.Fatalf("want a temporary 4xx reply with the panel down, got %v", err)
 	}
 	if err := checkContainerAlive(h); err != nil {
 		t.Fatalf("container did not survive the panel stopping: %v", err)