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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&#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!

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(NB: Since I haven't gotten any answers to my question, I'll try to answer it myself -- though based only on my own experience. If others have more knowledge/more experience, I'd still love to hear your thoughts and thus I'm leaving this question 'unanswered'.)

We've been sending emails via the AWS SendBulkTemplatedEmail action for several months now, which is unable to add custom mail headers such as the List-Unsubscribe header. So far, we've been pleased with the results with just one caveat (more on that later.) Our open and click through rates appear to be in line with what we experienced when adding the List-Unsubscribe header through the SES SendRawEmail action. So it seems like not including the List-Unsubscribe header hasn't affected our bulk mailings.

Well... Almost.

One thing we did notice was that a small amount (i.e., less than 1%) of our subscribers were showing excess clicks / opens in our reports -- and that these clicks / opens were occurring almost immediately after the emails were delivered. Which is to say that it seems likely to us that these clicks / opens were caused either by the receiving mail servers and/or the receiving mail clients and not by the actual human receiving them.

There were clear patterns in the domains of the subscribers for which this was an issue leading us to the conclusion that these artificial clicks / opens were being caused by MS Exchange and/or MS Outlook, possibly through the Safe Links feature of Microsoft Defender. To mitigate for this, we modified our software so that subscribers from these domains would be sent these emails with the List-Unsubscribe header by using the SendRawEmail action.

Our two-step approach seems to have solved the issue for us as we stopped getting artificially inflated clicks and opens in our reports. That said, we realize that if this is a client-side issue, the domain-based list we created may not be 100% effective. Still, the trade off is worth it for us, especially given that we are able to mail our list many, many times faster than we were able to before. And it should also be noted that though this phenomenon affected our click / open tracking, it did not seem to affect actual deliverability. (We observed many users with likely 'artificial' clicks / opens who later had what we believe to be real opens / clicks.)

TLDR: Not using the List-Unsubscribe header did not affect our deliverability, but did affect our click / open tracking statistics for a small amount of users, namely a subset of those likely using MS Exchange and/or MS Outlook.

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