Email notifications stopped spontaneously

I'm using the default email configuration.

SMTP host: mail 
SMTP port: 25 
Send email messages from: Teamcity 
SMTP login: <blank> 
SMTP password: <blank>
Secure connection: none

Worked fine for last year or so.  All of a sudden email notifications have stopped.  I've triple-checked the notification rules.  Nothing has changed.

Using Teamcity 7.1.1

Ideas?