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/milter/sender_test.go

@@ -94,6 +94,8 @@ func TestSenderCheck(t *testing.T) {
 		{"own domain, any address, with a name", "shop", [][2]string{from(`"The Shop" <Orders@Shop.Example.Org>`)}, true, ""},
 		{"second application of the domain", "shop2", [][2]string{from("x@shop.example.org")}, true, ""},
 		{"a listed address", "billing", [][2]string{from("Invoices <invoices@example.org>")}, true, ""},
+		{"a listed address with an extension", "billing", [][2]string{from("invoices+2026-march@example.org")}, true, ""},
+		{"an extension, any address of the domain", "shop", [][2]string{from("orders+48213@shop.example.org")}, true, ""},
 		{"header name in another case", "shop", [][2]string{{"FROM", "a@shop.example.org"}}, true, ""},
 		{"its own name inside the display name", "shop", [][2]string{from(`"shop@shop.example.org" <a@shop.example.org>`)}, true, ""},
 		{"an encoded display name", "shop", [][2]string{from("=?utf-8?b?0JzQsNCz0LDQt9C40L0=?= <a@shop.example.org>")}, true, ""},
@@ -107,6 +109,8 @@ func TestSenderCheck(t *testing.T) {
 		{"the server's own name", "shop", [][2]string{from("postmaster@mail.example.org")}, false, "not an address this application sends as"},
 		{"a stranger's domain", "shop", [][2]string{from("someone@gmail.example")}, false, "not an address this application sends as"},
 		{"an address off the list", "billing", [][2]string{from("director@example.org")}, false, "not an address this application sends as"},
+		{"an address off the list, with an extension", "billing", [][2]string{from("director+invoices@example.org")}, false, "not an address this application sends as"},
+		{"a listed name as the extension of another", "billing", [][2]string{from("+invoices@example.org")}, false, "not an address this application sends as"},
 		{"a subdomain of its own", "shop", [][2]string{from("a@sub.shop.example.org")}, false, "not an address this application sends as"},
 		{"no From: line", "shop", [][2]string{{"Subject", "hello"}}, false, "no From: line"},
 		{"two From: lines", "shop", [][2]string{from("a@shop.example.org"), from("director@example.org")}, false, "more than one From: line"},
@@ -254,3 +258,16 @@ func TestSenderCheckRefusalIsOneRow(t *testing.T) {
 		t.Fatalf("rows = %+v", rows)
 	}
 }
+
+// A message with the null envelope sender - a bounce an application writes
+// itself - is judged like any other, and its row belongs to the domain the
+// login is bound to.
+func TestSenderCheckRefusalWithNullSenderHasItsDomain(t *testing.T) {
+	bind := NewBindings(writeMaps(t, senderMaps))
+	for login, want := range map[string]string{"shop": "shop.example.org", "billing": "example.org"} {
+		_, rows := offer(t, bind, login, "", [2]string{"From", "someone@gmail.example"})
+		if len(rows) != 1 || rows[0].Domain != want || rows[0].From != "" {
+			t.Errorf("%s: rows = %+v, want one of domain %s", login, rows, want)
+		}
+	}
+}