Thanks to spammers, sending email is now a specialized activity, with email services like sendgrid and mailgun specializing in sending mail. It happens that MakerForums uses mailgun.
Hotmail is currently blocking all mail from mailgun that we are sending. We have opened a ticket for mailgun to fix their problem. In the meantime, you will not be able to create a new account using a Hotmail address. We believe that notification emails and digest summaries to established users are similarly affected.
If you are trying and failing to create an account with a Hotmail address, please try a different address, or send email to firstname.lastname@example.org to request help.