As a developer (working for Codeable among others) I’m looking for a way to inject additional CSS styles in the email messages.
Currently there seems to be no way to achieve this. I would like to suggest to add a filter hook at line 112 in mailpoet/lib/Newsletter/Renderer/Renderer.php like this:
$renderedStyles = WPFunctions::get()->applyFilters('mailpoet_rendered_styles', $this->renderStyles($styles));
This would allow developers to add some additional styling to the email messages.