Description
Transports like Postmark and Mailgun support message metadata and tags. The context of this issue will be Postmark.

#34766 implements tag support in Postmark by using a Tag header. This will work when using the Postmark API transport but not using the SMTP transport. For SMTP you need to add a custom header:

X-PM-Tag: password-reset

With the Postmark API, metadata is a key, value object added to the payload:

"Metadata": {
    "Color":"blue",
    "Client-Id":"12345"
}

With SMTP, it is a custom header for each value:

X-PM-Metadata-Color: blue
X-PM-Metadata-Client-Id: 12345

Example
Ideally, it would be nice to be able to do the following:

$email->setTag('password-reset');
$email->addMetadata('Color', 'blue');
$email->addMetadata('Client-Id, '12345');

Then the transports could convert to their specific tag/metadata format.

I can work on a PR for this if we can nail down how we want this to work.