Under Send backups by email I have my email that I receive other system notices at in the recipients list.
When clicking an envelope icon on a backup to do a manual send, the confirmation box pops up, I click Send Now and it says it sends. The notifier.log shows otherwise:
2014:02:11-12:20:32 www notifier[20959]: processing notification request for INFO-011
2014:02:11-12:20:32 www notifier[20959]: mail notifications for INFO-011 are disabled
2014:02:11-12:20:32 www notifier[20959]: successfully processed request for notification
I check under the Notifications -> Notifications tab, and there is no INFO-011 to enable (or disable).

The same behavior is observed if I try to send a backup using backup.plx as well.