Hello,
My team and I have been working on implementing some regularly scheduled emails for our clients using hubspot’s transactional email API + the beta custom coded template/module features.
These emails are really more like marketing emails, but to leverage hubspot’s single send API features we’ve been implementing them as transactional emails (there’s a lot of bespoke data that needs to be pulled into the emails per-client in our backend, and which we don’t want to store on HubSpot). Overall our desired implementation has gone well and the emails are almost ready to ship.
To get to the point, we’ve included unsubscribe links in the email footer which leverage our own unsubscribe solution, since these are going to act more like marketing emails. But as a final step we want to figure out how to implement List-Unsubscribe-Post and List-Unsubscribe headers in these emails, to comply with recently announced requirements from Google and Yahoo. So my question is, is modifying email headers (in general, and these ones specifically) supported with the transactional email / single-send API, and if so, can anyone give me direction about how to implement them (either in the email template itself or the API request we’re making to the single send API)? I’m having difficulty finding docs that describe how to implement this with the given API, have tried a number of ideas just to see whether something would work but nothing has worked out. Would appreciate any direction anyone can offer here. 
Thank you.
Edit: to make this a bit clearer, we’re sending an unsubscribe url which points to a domain of ours in the custom properties of our api request, and then including that in an unsubscribe hyperlink in the footer; such that when the user hits that endpoint it triggers unsubscribe logic on our side. We want to also include this in a header. This is important to us because we want our clients to be able to manage their subscription status both in-platform (ours) and through clicking the unsubscribe link, hence the custom solution here.
