All email notifications have stopped


1. I have four identical WordPress sites with the same plugins.

2. I use Jetpack's Blog Subscriptions.

3. I have stopped receiving email notifications of any new subscribers, user registrations, comments, etc.

4. I installed the Check Email plugin and did not receive any of the test emails from any of my four sites.

5. I deactivated all of the plugins on one site, reactivated the Check Email plugin and sent a test email, which I did not receive.

Any advice or suggestions?