Had that problem myself for weeks, too. Subscribers with Microsoft free email accounts (hotmail.com, live.com, etc) did not get confirmation emails, not even in their spam folders. The reason is that we are hosted on a shared virtual server and it was registered as a spamming IP-address (not because of us but because of another former user of that server) by Microsoft, so they blocked emails coming from our server even before people could get them in their email clients.