The generation of email HTML is dependent on newsletter API keys being inputted. If you use Zapier and a third party without the API keys being inputted, email HTML (the full one) that can be embedded to say Mailchimp or any other third party is not generated. Instead, there is a reduced version that loses formatting or plaintext.
The obvious solution is to input fake keys, mine is 12345, and it will indeed fail, but it is enough to generate the email.
Hopefully, this can be made to be the default whether a mailer is provided or not.