How to get custom variables when using a custom Email Transport with CakePHP 4, for sending bulk mails with Postmark

Question

I want to send bulk mail (newsletters) with the Postmark Batch API. Postmark Batch API requires me to combine up to 500 mails per request.

Therefor I have created an custom NewslettersTransport.php which is handeling the API call and setting up the batch request. So far so good.

The content of the mails should be personalized. I am looking for a way to do this inside this Transport method, but I can't access my ViewVars set in NewsletterMailer.php

I'm searching for a different logic or a way to access the ViewVars / customize the message body.

// NewsletterMailer.php

return $this
    ->setEmailFormat('html')
    ->setTo($subscribers->all()->toArray())
    ->setFrom([$newsletter->sender => $company->companyname])
    ->setSubject($newsletter->name)
    ->setProfile('newsletter')
    ->setViewVars([
        'company' => $company
    ])
    ->viewBuilder()
    ->setTemplate('newsletter_mail')
    ->setLayout('default');