would be great to add a test to prevent regressions

Hm, since the header type is passed as string...

$header = new UnstructuredHeader('Subject', 'Test');

...how can a regression be tested?
Create a new test for the entries in private const HEADER_CLASS_MAP?

@ThomasLandauer create an Email with a In-Reply-To header (or a References header for the other test to add) being a value that does not respect the msg-id spec, i.e. exactly what fails with the current code of the component.

OK, thanks - like this?

@nicolas-grekas @fabpot based on the issue being solved, I suggest we consider this as a bugfix.

Works for me

Thank you @ThomasLandauer.

@nicolas-grekas Isn't this a BC break? The userland code has to be changed when updating from 5.4.1 to 5.4.2 (use the text header and add the brackets yourself).

Before:

$headers->addIdHeader('References', $messageIds);
$headers->addIdHeader('In-Reply-To', $messageId);

After:

$headers->addTextHeader('References', '<' . implode('> <', $messageIds) . '>');
$headers->addTextHeader('In-Reply-To', '<' . $messageId. '>');

Please open issues if you want further discussion.

Yeah, you're right, this might be a BC break - sorry about that!
You can solve it by changing ->addIdHeader() to the generic ->addHeader()

Yes, you can use the more generic addHeader method. But then you also have to remember to add the brackets to each message id yourself, as they are only added with the IdentificationHeader and not with the UnstructuredHeader.