The last incriminating bit of evidence has now been obtained (Thanks, Carlos ... ), and we know what caused the outgoing notifications to get shut off ...
Lordmongrove did it ...
His posting of a story linked to hindustantimes.com triggered the release of notifications, after which the security checkers at our outboard email service flagged the reference to a known spam / mischief point of origin as a potential problem and cut off our service as a precaution.
Our tech lead is in negotiations with our email service provider to see what can be done to resume notifications.
This is shaping up to entail additional security measures (e.g., CAPTCHA) on our email traffic, and it may take some time to get everything settled.