e mail programing project, what is the key abstractions in the system as objects or object classes

Hello this is my use case of e mail programing project, what is the key abstractions in the system as objects or object classes (in java) Can you help me please ?

I know I can have any number of objects , in case of email programming project the objects are as such, system admin, system user and they have many related domains but what are those ? And can you write the class in java ? Thank you

Create Mail Groups system admin Manage Users Manage MailBox and Domain Names Send Mails Manage Addresses Login/Logout Update and Delete Account system user Receive Mails user Message Data Delete Mails UserData