At the end of August, Microsoft took the decision to change how they handled emails and dramatically strengthened the requirements before they would allow people to email their users. This caught everyone by surprise, and immediately all the Hotmail users, as well as msn.com, outlook.com and a number of other users haven't been getting any emails from us about their followed topics, PMs and so on. This has also affected new members, who are unable to receive their validation emails.
We've been working to get this resolved as soon as possible, but as you might suspect with these things, sometimes it takess a long time, especially as they're no doubt receiving a TON of requests for removal from their blacklist around now (can't think why?), but we had an email this afternoon that implied we'd be being unblocked in the next 24-48 hours, which has been confirmed by a number of members now
To anyone thinking of signing up with any of those email providers, please give consideration to using a different account so that you can get your validation and notification emails in future. Who knows when/if they'll do it again?