It could happen that the directory sync service (DirSync) doesn’t sync the users UserPrincipalName correctly.
I had an issue where the UserPrincipalName from a user in the Office 365 windows azure directory has been made based on the user’s sAMAccountname. This wouldn’t be problem if as long the sAMAccountname is the as same as the UserPrincipalName, but as you can guess this is not everywhere the case.
In certain cases SharePoint alerts are more useful than workflows, f.e. having the possibility to let users manager their alerts is not possible with workflows.
To manager alerts for a couple of users on specific lists I’ve written a script that handles the whole cycle of alerts. It let me delete, update and create alerts based on predefined configuration.
When deploying a published SharePoint 2013 on-premise installation or a Office 365 installation or a SharePoint Online installation, it’s highly recommended to update your Microsoft Office 2013 installation.