Symfony version(s) affected: 4.4.10, but also present in master

Description
In-Reply-To and References headers are validated as ID-Headers: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Mime/Header/Headers.php#L37
...which leads to validating them as Address: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Mime/Header/IdentificationHeader.php#L86

Yes, this is exactly what RFC 2822 is saying.

However, I'm suggesting to loosen the validation.

Why? When replying to an email, the original email's Message-ID goes into In-Reply-To or References. This means: If the value doesn't adhere to the msg-id spec, I can't do anything about it! The only choice I'm having is to include it as-is, or omit the header entirely. And IMO including it is the by far better choice, since it gives the recipient's MUA the possibility to correctly append it to an existing thread.