SMTP backend crashes on invalid email addresses such as “to@”

[Kwadwo Owusu Ansah]

While running the Django test suite on the current development version
(6.1.dev), the SMTP backend crashes when preparing invalid email
addresses such as "to@".

This failure is triggered by the test:
tests/mail/tests.py::test_avoids_sending_to_invalid_addresses

Expected behavior:
Invalid email addresses should be rejected early with a ValueError and
must not reach the low-level email header parser used by
AddressHeader.value_parser().

Actual behavior:
The following exception occurs when attempting to parse the invalid
address:

IndexError: string index out of range

Traceback (excerpt):

File django/core/mail/backends/smtp.py, in prep_address

parsed = AddressHeader.value_parser(address)

File .../email/_header_value_parser.py

if value[0] in CFWS_LEADER:

IndexError: string index out of range

This indicates that the backend attempts to parse malformed email
addresses too deeply instead of validating them first.

Environment:

• Django version: 6.1.dev
• Python 3.12
• Command: python tests/runtests.py mail

Steps to reproduce:

1. Clone Django main
2. Install test dependencies
3. Run: python tests/runtests.py mail
4. Observe the failure in test_avoids_sending_to_invalid_addresses

The failure is reproducible and appears to be a regression in the SMTP
backend’s handling of invalid recipient addresses.

[Kwadwo Owusu Ansah]

Patch submitted in ​https://github.com/django/django/pull/20301.