How do you handle unsubscribing?
Though this sounds like a feature of a more managed solution. I know MailChimp has this out of the box (and possibly other providers).
Since this library is supposed to provide email automation capability, I'm not sure options like Unsubscribe should be part of the library.
What do you think? Definitely open to ideas.
I can't think of a quick and easy way to do it in your lib right now, but this kind of thing is pretty much why I use MailChimp/Sendgrid/etc. for even tiny projects.