Force emails to stack in Outlook

I have a setup where I send out emails, if a member creates/posts in a topic. When a new topic or post is created all members who subscribe will be informed by an email, which they can reply to directly post in the given topic. This is working as a charm. I have a node backend, with nodemailer and Mailgun as email client, where I use the nodemailer Mailgun transport layer.

Some of my members are starting to complain about emails are not stacking in their respective clients. The issue was easy to solve for gmail, because gmail stack emails with the same subject.

But members who use Outlook, don't get nice stacked emails based on topic. I've read a bit about the issue, and I need to set the "In-Reply-To" and "References". I have tried to add these values like this:

this._nodemailerMailgun.sendMail({
    inReplyTo: topicId,
    from: from,
    to: to,
    subject: subject,
    html: htmlContent,
}, (err, info) => {
    if (err) {
        winston.error(err);
    }
    winston.log('info', 'mail send to: ' + to);             
});

But I'm not sure if the Mailgun transportlayer, is reading the inReplyTo property? Is it at all possible to force stacking in Outlook with my current setup?

And I would be very graceful if someone could tell me or give me a link, to how Outlook stacks emails.

via DNRN