Mimemail runs all http headers through UnstructuredHeader() while it should not

Well no, an object is not a valid value for the message array. In the Drupal API this array is defined to be the same as the PHP mail() function arguments. What you should do in your hook is convert your date to a string.

Unfortunately you are wrong.

I suggest you test it without the mimemail module enabled, so just using Drupal Core PHP Mailer:

1. Send email with 'Date' email header set to DateTimeInterface $message['headers']['Date'] = new DateTime();. The email will send correctly.

2. Now send another email but this set 'Date header to any string date, like a result of date('r') $message['headers']['Date'] = date('r'); You will get this error:

TypeError: Symfony\Component\Mime\Header\Headers::addDateHeader(): Argument #2 ($dateTime) must be of type DateTimeInterface, string given, called in \vendor\symfony\mime\Header\Headers.php on line 152 in Symfony\Component\Mime\Header\Headers->addDateHeader() (line 114 of \vendor\symfony\mime\Header\Headers.php).

Symfony\Component\Mime\Header\Headers->addHeader('Date', 'Wed, 13 Dec 2023 08:29:21 +0100') (Line: 102)
Drupal\Core\Mail\Plugin\Mail\PhpMail->mail(Array) (Line: 50)
Drupal\mailsystem\Adapter->mail(Array) (Line: 307)
Drupal\Core\Mail\MailManager->doMail('mymodule', 'mykey', 'user@example.com', 'pl', Array, NULL, 1) (Line: 180)
Drupal\Core\Mail\MailManager->Drupal\Core\Mail\{closure}() (Line: 592)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 181)
Drupal\Core\Mail\MailManager->mail('mymodule', 'mykey', 'user@example.com', 'pl', Array, NULL, 1) (Line: 70)

As you can see a DateTimeInterface object is expected, NOT a string.

Can we have this resolved please?

How can I help further, despite the patch provided, to have this fixed? If you don't have time then add me as a maintainer and I will fix it.

@pawel.traczynski starting a reply with

Unfortunately you are wrong.

is not a great way to start a constructive discussion, I'd bet @TR stopped reading after that line... doesn't matter how correct you may or may not be.

I have an Merge Request here https://www.drupal.org/project/mimemail/issues/3257799#comment-15486431 🐛 RfcComplianceException: RFC 2822 Needs work that might inadvertently do what you are asking for, give it a try if you agree with the approach.

A couple things on the issue correct: all patches should be applied against the dev branch, so changing the version to the dev branch. And since there is a patch on the issue it's good to have the status as "Needs Review" (also used to trigger tests to run against the patch)

Thanks @joelpittet. I found the first reply starting with "Well, no" to not be good start of a constructive conversation either but I see the point. I have provided patch and solution. Has it been integrated into the module?