1/ If you send an email via 'administration' > 'Send email', then the selected email-address will be the one that is related to the person.
2/ If you share an object (task, document, appointment, etc), and select to notify the user with an email, then this notification will be sent to the email account related to the user:
General info: user functionalities depend on the roles & permissions settings. If a certain functionality does not work within your user account, this will be because you don't have sufficient permissions for this.