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>

internal/store/sendercheck.go

@@ -88,13 +88,26 @@ func (s *Store) AddSenderCheckDeferred(at time.Time, n int64) error {
 	if err != nil {
 		return err
 	}
-	stamp := at.UTC().Format(time.RFC3339)
+	// To the fraction of a second the log gives, and of a fixed width so that
+	// the strings compare as the times do.
+	stamp := at.UTC().Format("2006-01-02T15:04:05.000000Z")
 	if stamp > last {
 		return s.SetSetting(settingSenderCheckDeferredAt, stamp)
 	}
 	return nil
 }
 
+// SenderCheckDeferredAt is when a message was last counted as deferred; zero
+// if none was.
+func (s *Store) SenderCheckDeferredAt() (time.Time, error) {
+	v, err := s.GetSetting(settingSenderCheckDeferredAt)
+	if err != nil || v == "" {
+		return time.Time{}, err
+	}
+	at, _ := time.Parse(time.RFC3339Nano, v)
+	return at, nil
+}
+
 // SenderCheckStats sums the days up to now: today, the last 30 days and the
 // last 12 months, each including today.
 func (s *Store) SenderCheckStats(now time.Time) (SenderCheckStats, error) {