This question has been asked in many guises over time, and to some extent ignored, but I'll ask it another way.
e-newsletter seems to have a pretty custom subscription service,people have been asking for 'form action' code, so they can get the subscription working from other methods rather than just the shortcode or widget. I have not yet seen an answer.
What I would love to be able to do (as is possible with MailChimp for instance & CF7) is to have a 'tick box' on my contact form to opt-in when making contact. In my particular case my contact form is Gravity Forms, and whilst I can 'hook' code into the submit, without some form of 'API' I'm all a bit stuck.