EmailMessage repeats header if provided via the headers kwargs

[Aalekh Patel]

If you create an EmailMessage instance with a "To" key in the headers= kwarg, it attaches the To header to the email two times, violating ​RFC 5322#3.6.

My suspicion is that it attaches it the first time from extra_headers in self._set_list_header_if_not_empty(msg, 'To', self.to) at ​django.core.mail.message:266 and the second time again from extra_headers at ​django.core.mail.message:282

        message = EmailMessage(
            subject="test subject",
            body="test body",
            from_email="from@example.com",
            to=["guy1@example.com"],
            headers={
                "To": ", ".join(["guy1@example.com", "guy2@example.com", "guy3@example.com"]),
            },
        )

For example, here is a Python 3.9.18 shell output for the EmailMessage above that shows the To header appears twice.

>>> from django.core.mail import EmailMessage

>>> message = EmailMessage(subject="test subject", body="test body", from_email="from@example.com",to=["guy1@example.com"], headers={"To": ", ".join(["guy1@example.com", "guy2@example.com", "guy3@example.com"])})

>>> print(list(message.message().raw_items()))
[('Content-Type', 'text/plain; charset="utf-8"'), ('MIME-Version', '1.0'), ('Content-Transfer-Encoding', '7bit'), ('Subject', 'test subject'), ('From', 'from@example.com'), ('To', 'guy1@example.com, guy2@example.com, guy3@example.com'), ('Date', 'Wed, 13 Dec 2023 15:59:31 -0000'), ('Message-ID', '<170248317136.759.5778419642073676754@036d358ca984>'), ('To', 'guy1@example.com, guy2@example.com, guy3@example.com')]

I've provided a patch for this here: ​django/django#17606

[Natalia Bidart]

Reproduced, and accepting based on the RFC which states:

   +----------------+--------+------------+----------------------------+
   | Field          | Min    | Max number | Notes                      |
   |                | number |            |                            |
   +----------------+--------+------------+----------------------------+
   | to             | 0      | 1          |                            |

This is related to #9233, and I would encourage for the solution to this ticket to cover for all those headers that should provide at most 1 occurrence.

[Adam Johnson]

Please don’t CC me on tickets I have no relation to.

[Adam Johnson]

Sorry, I thought the CC was done by OP. It happens sometimes that people just ping me because they’ve seen my blog or something. Why did you add me Natalia? I don’t remember working on emails 😅

[Natalia Bidart]

Hey Adam, I should have been more explicit, my bad! I CC'd you because I read your comment in ticket:32907#comment:1 and it seemed that you were interested/knowledgeable in the topic. Sorry if that was hasty!

[Florian Apolloner]

Should we override with the data in headers or should be throw an exception? As the behavior is broken now an exception wouldn't be the worst thing and feels more correct (ie use the existing API to set to because that might do other things under the hood etc…)

[Adam Johnson]

I agree that an exception sounds more correct, probably a ValueError, for any keys in headers that correspond to the explicit arguments like “to”.

Natalia - no worries, I just saw this message after a number of other spammy ones 😅

[Salvo Polizzi]

Hi, i made a PR for this bug: ​https://github.com/django/django/pull/17606/files

[David Wobrock]

This is the PR you meant ​https://github.com/django/django/pull/17642, right? :)

[Salvo Polizzi]

Replying to David Wobrock:

This is the PR you meant ​https://github.com/django/django/pull/17642, right? :)

Yes, it is

[Florian Apolloner]

I do not see any tests in the PR, just because no tests fail after changes doesn't mean there are no tests required ;)

[Florian Apolloner]

When working on the patch, please consider the history of the file: ​https://github.com/django/django/commit/5e75678c8b was added to ensure that to in extra_headers takes precedence and should have prevented a duplicate to header. Apparently this got broken in ​https://github.com/django/django/commit/da82939e5a31dea21a4f4d5085dfcd449fcbed3a

Whatever the final solution is (raising an error if possible is preferred -- I fear it is not), existing tests and usecases shouldn't break.

[Salvo Polizzi]

Replying to Florian Apolloner:

I do not see any tests in the PR, just because no tests fail after changes doesn't mean there are no tests required ;)

I'm sorry, I will provide tests. Thanks for giving me some feedback

[Salvo Polizzi]

Replying to Mariusz Felisiak:
Hi Mariusz, what do i need to fix in the PR? I'm asking you because I haven't seen any reviews in the PR anymore.

[Mariusz Felisiak]

Sorry, I forgot to click "Submit the review".

[Salvo Polizzi]

I left some comments in the new PR: ​https://github.com/django/django/pull/17674

[Mariusz Felisiak]

We need to find ​answers before this PR will be reviewable again. We cannot review PR if we are not sure what we want to achieve. I don't see any discussion on the forum or the mailing list, or any answers for my questions.

[Natalia Bidart]

​Forum post

[Mike Edmunds]

Note: #9214 added special handling for supplying from_email=... with a different headers={"From": ...}: the header value is displayed in the message, the from_email value is used as envelope-from/return-path.

I can't find the reference now, but I believe using to=... with a different headers={"To": ...} was added around the same time and has a similar purpose: specifying the recipient separately from the displayed recipient. (This is sometimes used for distribution lists, where the list name is displayed in the header To field. It's also used for spam.)

In both cases, the headers value needs to override the property value in the generated message header. (Not create an additional header.)

It seems like there might also be missing tests for these special cases?

[django-anymail maintainer here; a few years back, we got a ​specific request to match the Django SMTPBackend's handling of these headers.]

[Mike Edmunds]

Ah, the forum mentions the ticket I couldn't find: #17444 allowed different to=... and headers={"To": ...}

I think this got broken when EmailMessage._set_list_header_if_not_empty() was added. And cc and reply_to are likely broken in the same way (based on reading the code).

The problem is that Python's email.message.Message ​behaves as a multi-value dict when assigning to header keys, but ​returns only one of the values when reading from keys. The current logic in _set_list_header_if_not_empty() assumes it works like a regular dict, and the tests from #17444 were insufficient to catch that mistake.

The minimal and safe fix is something like:

1. First make the existing tests fail on current buggy behavior: update ​test_to_header() to use ​get_all("To") and verify there's only the one correct value in the list. Same thing in ​test_reply_to_header(). Wouldn't hurt to update some other nearby tests in the same way, and add a test_cc_header().

2. Then fix ​this line in _set_list_header_if_not_empty() so it only assigns to the header if there's not already a header there. (Indent it one level so it only runs in the except clause.)

I would suggest we not try to enforce ​RFC 5322-3.6 email header counting rules in Django. If/when Django ​moves to Python's modern email API, those (and many other email header restrictions) will be enforced by Python.

[Mike Edmunds]

(Also, that minimal and safe fix should be easy pickings)

[Mike Edmunds]

The problem originally reported in this ticket was a regression introduced in #28912 ​PR #9454.

I've added a new patch ​PR #18325 which addresses that specific regression. (Without attempting to add some of the other enhancements and email RFC enforcement contemplated by other discussion here.)

[Sarah Boyce <42296566+sarahboyce@…>]

In b909853:

Fixed #35033, Refs #28912 -- Fixed repeated headers in EmailMessage.

Fixed a regression which would cause multiple To, Cc, and
Reply-To headers in the result of EmailMessage.message() if
values were supplied for both to/cc/reply_to and the
corresponding extra_headers fields.

Updated related tests to check the generated message() has
exactly one of each expected header using get_all().

Regression in b03d5002955256c4b3ed7cfae5150eb79c0eb97e.