To allow managers and employees to receive notification emails of actions such as time off requests you need to ensure that the "Work Email"  field contains the correct value.

This can be found by browsing to the employee card, selecting "Edit" and clicking on the "Job Details" icon found near the top of the screen.

People -> Employees -> Edit -> Job Details

Even though when setting up a Company user account you may enter an email address as the login name, this value is not used as when a users email address is updated in the employee file this would then mean the two values would be out of sync so we always use the "work email" value for system notifications.