Is the problem with ALL subscriptions now?
When I encountered the issue a while back (obviously ongoing) I tracked it back to MSN/Hotmail blocking enworlds thread notifications as spam. They don't even send it to my spam filter, they apparently just kill some emails and don't even send me it even when I set them as a safe sender.
If that's the issue, I think the only way to 'really' fix is it to talk with MSN and see what they want in order to unblock it. I have no idea how difficult that would be. However, you can workaround by changing your email address to a gmail account or anything else that's working.
If the issue though is with all emails... well that's another issue. Any idea how far the mail is getting out before it dies?