You can set the AD_User => NotificationType to 'X' at both iDempiere and at the database and the field will still get set to 'EMail' in some cases.
Times when NotificationType will default to 'EMail':
When the AD_User record is saved through the model and the NotificationType is not explicitly set. Note that the constructor defaults to EMail.
When the AD_User record is saved through the QuickEntry option and the NotificationType is not explicitly added to the list of QuickEntry fields