Some Email Notifications fail to send, if one of the users in the list of users receiving them has an invalid email address

Symptoms:

Some users fail to receive some page, or blog post notifications. It seems to happen sporadically, but consistently for a specific space.

Potential cause:

The notifications plugin queues up emails with a retry mechanism. This retry attempts to re-send to a recipient when failure happens for whatever reason. This includes failure to verify the email address validity (e.g., a user has a blank, or empty email address).

The retry does not remove the currently failing recipient, and re-queues the notification, causing it to fail next time (for the same reason). After a set number of failures, the entire batch of recipients is discarded, and thus some recipients (who does have valid email address) fail to receive any email.

Workaround:
Find the offending user (user with no/invalid email address) by going to the Manage watcher dialog on the page, and remove them, or add a valid email address.