Users automatically unsubscribing

I have two clients who I've manually added (with their permission). (I actually have many clients, but just 2 are having this issue).

I created their accounts and marked them as confirmed.
I added them to several lists.
I create a newsletter to go to one of the lists that they are subscribed to.
I manually send the newsletter.

EXPECTED RESULTS:
Emails sent to everyone in the list.

ACTUAL RESULTS:
Within seconds of getting the NS Pro Notification email (that the email was sent), I get an email saying that these individuals have unsubscribed.
The email IS delivered to the recipients.
The 2 (who are using Office365) are removed from all list subscriptions and can't be found in NS Pro subscribers list.

VERSIONS:
Joomla: 3.10.14 (Yes, I know, I'm in the process of upgrading ... and the email I'm sending to these clients was about that process)
PHP: 8.1.26
DB: Maria 11.1.3
NSPro: 3.0.2428

OTHER INFO:
When I add the user as a subscriber, they appear in the nspro_subs, but as soon as I send and that unsubscribe happens it is gone.

This appears to be happening only for users who have Office365 email service.

Originally I had the Double Opt-In set to YES (because I want subscribers on the site to opt in).
When I set this to NO, and run the process above (i.e. send the email) the user did NOT get unsubscribed at first. But after a 2nd send, they are unsubscribed from all lists.



NOTE: I tried to post this on the forum once before but the reCaptcha just spins and spins there after checking it. Can you please disable captcha for logged-in users? It really is a pain if we are already a client

Well, I wouldn't think that it is. We are using Amazon SES for outbound sending on this system. And the accounts are all confirmed (and using SPK/DKIM) and authenticated.

I do know that one of them is in a corporate environment where they have a lot of rules set on their email system. But they have white-listed our account.
And Amazon SES is set-up to deliver with our domain as the authenticated from address.

We won't be able to access their email accounts directly, but they do have a pretty skilled IT dept. so I'll push it back on them to do some more checking.
I might also try using another bulk email component and compare the results as well as the contents of the email and see if I can figure out what the problem might be.

Is there a way to disable the automatic insert of the unsubscribe data that is added to the email? (We already include that in the footer of every email we sent). Just wondering if that might be mis-interprested by Office365.

Well, I just sent the same email content, using the same email server, from the same Joomla site.
One with NSPro, one with AcyMailing.
The AcyMailing email had no issues (the user didn't get unsubscribed).
The NSPro email always ends up with an unsubscription.
This is happening with both users who have Office 365.

I've looked over the email source on both and the only main thing that I can see is that NSPro includes the line:

List-Unsubscribe: https://<<domain>>/index.php?option=com_nspro&task=confirm_unsub&u=5dbc15ce3af94193ef073dd1e50a226c

If you can tell me where that unsubscribe line is being added (which file, which line of code) I can do a test by removing that and seeing if that resolves the issue.