I have a similar but not identical problem. I installed this for a client, there is a dedicated Subscribe page where the S2 form is visible. When I subscribe with an e-mail address at hotmail.com confirmation notice does not come at all. When I subscribe with a live.com e-mail confirmation mail comes hours, perhaps overnight late.
However, when I subscribe with my private email or a .edu address the confirmation mail for subscribe or unsubscribe arrive instantly. I am not sure how to proceed.
The sending of mails is from a user whose e-mail is on the same domain, neit.edu and it is able to send mail to some domains but not others. Yet, a third domain gets the mail but with a great delay.
You can check the behavior at:
you will see the Subscribe page on the menu. Any direction to resolve this issue will be much appreciated.