Question: How important is it to include a 'List-Unsubscribe' header when sending bulk email?
Background: Our website has a fairly large email list (roughly 275,000 subscribers) that we email every two weeks or so. We've worked hard to ensure that the both the list and the emails we send are of the highest quality and that all our emailing practices are both ethically and technically to the highest standards. We currently use AWS Simple Email Service to mail our list, which has been great for the most part; however, as our list has grown, our email can take a very, very long time to send to the entire list. We've done some tests, and it's apparent that the biggest issue we face is that the size of our messages is fairly large (100KB per message) and so it just takes a decent amount of time to transmit that data to SES from our own servers.
We recently discovered that SES offers a bulk sending API (see: https://aws.amazon.com/blogs/messaging-and-targeting/introducing-email-templates-and-bulk-sending/), which looks like it could dramatically speed up our list mailing process (and save us some money too!) The problem is that it doesn't appear that there is any way to add a 'List-Unsubscribe" header to messages sent using this method (see: https://forums.aws.amazon.com/thread.jspa?messageID=873400󕎸).
So again, I'm wondering how important that list-unsubscribe header really is? Admittedly, while we're great at content and pretty good at the technical stuff, we don't know much about email deliverability aside from what we've learned doing web searches.
Will our messages start getting flagged as spam if we don't include it? We already try to make it as easy as we can for users to unsubscribe (including unsubscribe links in both the header and footer of the email), but is this header important from a usability standpoint? Will our own or SES's reputation help to mitigate any potential deliverability issues? Are there other considerations we should think about before embarking on a path where we no longer include this header?
Thanks in advance for any advice you might have!