Plesk should recognize the failure of backup at FTP Storage as an emailable error.
In Scheduled Backup settings, you can tick; "if errors occur during the execution of this backup tasks send a notification email to...". However, if the backup is set up to go to FTP Storage and the FTP site can't be reached, the backup is done to the Server Storage and no error email is sent. This is a problem because if a few days of backups intended for FTP Storage fail and go to Server Storage, contracted server space could be exceeded causing Plesk to suspend / disable the site.
Plesk should complete the backup to the Server Storage, but recognize the failure of the backup going to the FTP Storage as an error worthy of an email notification.
Thank you for your input! We will consider this functionality in upcoming releases, if it will be popular.
Everyone, please continue voting for this feature if you consider it important.