sender check: the sender map read and looked up as Postfix does it

Postfix 3.7.11 takes the owners of the first key mail_addr_find() finds -
the address, then without its extension, then @domain, each tried only
when the one before is absent (src/global/mail_addr_find.c,
mail_addr_find_opt()) - and compares the login with them by
strcasecmp_utf8() (src/smtpd/smtpd_check.c,
reject_auth_sender_login_mismatch()). The check accepted a From: address
when any of the three keys named the application: an application holding
@domain could write an address listed for another application of the
domain, which Postfix refuses it in the envelope, and have it signed
(assumptions B-PS1).

The table is now read as dict_thash_open() reads it: continuation lines,
owners split on commas and blanks (B-PS4), the key ending at the first
blank outside quotes, keys folded; a key given twice makes Postfix refuse
the whole table (no DICT_FLAG_DUP_* flag), and the check then defers too.
The writer folds keys, so two cases of one address cannot be two lines.

e2e on the stand (2026-10-14, make e2e, exit 0): the wildcard application
is refused billing@ - listed for app-list - in From: (550 5.7.1), and in
the envelope by Postfix at RCPT ("553 5.7.1 <billing@...>: Sender address
rejected: not owned by user"). go vet and go test ./... pass on the
server (23 packages).

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

internal/postfix/postfix_test.go

@@ -50,6 +50,22 @@ func TestRenderSenderLoginMapsDedupesLogin(t *testing.T) {
 	}
 }
 
+// Keys differing only in case are one key to Postfix, which folds them, and
+// two of them would make it refuse the whole table (dict_thash_open() with no
+// DICT_FLAG_DUP_* flag): they are written as one line.
+func TestRenderSenderLoginMapsFoldsKeys(t *testing.T) {
+	got, err := renderSenderLoginMaps([]Binding{
+		{"Billing@Example.org", "billing"},
+		{"billing@example.org", "billing2"},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "billing@example.org billing,billing2\n" {
+		t.Errorf("map = %q, want one folded key", got)
+	}
+}
+
 func TestAssertMapSafeRejectsInjection(t *testing.T) {
 	bad := []struct{ addr, login string }{
 		{"@exa mple.com", "log"},