By default, all emails sent to employees from the system will go to their work email address. This only changes when an employee becomes a leaver, when the termination email containing their login details will go to their personal email address instead.
If someone is a current employee and they are getting emails to their personal email address this means they do not have a work email added.
To resolve this:
Go to their profile, in the Employment tab.
Click on Position in the menu on the right.
Scroll down to Work Contact and click on Edit.
Change the work email address as needed and click Save.