from-header-binding: what the reviews had left as found, and address extensions

The owner on the three things the reviews' commit left as found: can they be
fixed? They can.

- Downtime that crossed a UTC midnight was all booked on the day of the
  restart: it is cut at the midnights between (byDay).
- A message with the null envelope sender - a bounce an application writes
  itself - left a refusal row without a domain, which no domain
  administrator would see: the row takes the domain the login is bound to.
- A log read again after an unclean stop counted its deferrals again: while
  the tailer catches up, a warning no later than the last one counted is
  passed over. The mark is kept to the fraction of a second the log gives.

And on the check being stricter than Postfix for an address with an
extension (invoices+x@ passed in the envelope, was refused in the header):
it makes Postfix's second try now, the address without its extension.
recipient_delimiter was the distribution's default; it is set in
postfix-config.sh, since the check depends on it. e2e sends one through.

Test server, this tree: go vet ok, go test 23 packages ok, make e2e ok (339 s).

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

cmd/panel/sendercheck_test.go

@@ -45,3 +45,24 @@ func TestSenderCheckDowntime(t *testing.T) {
 		t.Errorf("a mark in the future: %s", d)
 	}
 }
+
+// Downtime is booked on the days it fell on.
+func TestByDay(t *testing.T) {
+	at := func(d, h, m int) time.Time { return time.Date(2026, 10, d, h, m, 0, 0, time.UTC) }
+	got := byDay(at(12, 23, 50), at(14, 0, 10))
+	want := []dayPart{{at(12, 23, 50), 10 * time.Minute}, {at(13, 0, 0), 24 * time.Hour}, {at(14, 0, 0), 10 * time.Minute}}
+	if len(got) != len(want) {
+		t.Fatalf("parts = %+v", got)
+	}
+	for i := range want {
+		if !got[i].day.Equal(want[i].day) || got[i].down != want[i].down {
+			t.Errorf("part %d = %+v, want %+v", i, got[i], want[i])
+		}
+	}
+	if one := byDay(at(12, 10, 0), at(12, 10, 0).Add(40*time.Second)); len(one) != 1 || one[0].down != 40*time.Second {
+		t.Errorf("within a day: %+v", one)
+	}
+	if none := byDay(at(12, 10, 0), at(12, 10, 0)); len(none) != 0 {
+		t.Errorf("no time: %+v", none)
+	}
+}