Looking at the code PR (symfony/symfony#48901), I don't fully understand the description or the code/tests.

@m42e could you please show some scenario where this feature would be useful and a short example of how to use it? Thanks!

@javiereguiluz
The major use-case is anonaddy when forwarding a mail that already has the ids in the text. The workaround would be to replace the ids with a name, which is afterwards replaced by an id again.

Thanks. Can you share a real example of using this feature? Of course, if you prefer, you can submit a PR yourself.

Here is an example, stolen from https://github.com/anonaddy/anonaddy/blob/4f47bc4f4cf15e20555e99290e09cc85e82691ba/app/Mail/ForwardEmail.php#L232

foreach ($this->emailInlineAttachments as $attachment) {
                        $part = new InlineImagePart(base64_decode($attachment['stream']), base64_decode($attachment['file_name']), base64_decode($attachment['mime']));

                        $part->asInline();

                        $part->setContentId(base64_decode($attachment['contentId']));
                        $part->setFileName(base64_decode($attachment['file_name']));

                        $message->attachPart($part);
                    }

It is using the content_id as part identification and it does not touch the body of the mail message. The original message content will still reference the content_id

Does this help? I don|t know how to better describe it.

@m42e I tried to document this PR behavior in #18817

I think we shouldn't document you're use case but be more generic.

Review is welcomed 😄