I'm not sure exactly what time they'll come back. EDIT: They do now seem to be back...
But the lack of e-mail service highlighted a massive bug which resulted in a server crash when trying to send notifications. I thought I had found them all yesterday but discovered that when leaving comments still crashed when trying to send notifies. The bug was in the error handling of a failed send. So because the email service is normally so reliable I hadn't noticed that my error handling wasn't correct.
It is now rectified and shouldn't be seen again, especially when the mail service resumes later today.
Apologies for the lack of notifications today.
And a Happy New Year!