Activation emails are currently not being delivered to Microsoft related accounts, including Outlook & Hotmail.
To activate my account, I had to create this second account using a gmail address.
Microsoft policies are more strict every day, I personally had the same problem few months ago with a web application I created using sendgrid to deliver emails.
I ended up switching to sendinblue with a private ip, only at that time I was able to deliver to MS accounts. In my case, Microsoft was rejecting the email at all, not even delivering to junk/spam, and that is exactly what is happening here.
Please check because some users may end up giving up registering, after not receiving the email…