Invoice email transmission: Handle permanent failures (Z#23205576) (#5420)

* Invoice email transmission: Handle permanent failures (Z#23205576)

* Add missing raise branch

* Fix missing file

* Fix missing license header

src/pretix/testutils/mail.py

@@ -0,0 +1,31 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+import smtplib
+
+from django.core.mail.backends.locmem import EmailBackend
+
+
+class FailingEmailBackend(EmailBackend):
+    def send_messages(self, email_messages):
+        raise smtplib.SMTPRecipientsRefused({
+            'recipient@example.org': (450, b'Recipient unknown')
+        })