What's the difference between a mailbox and a user name?

The email address is unique, just like your phone number, it is a domain name, and others need to know your email address when writing to you.

A user name is just like your name in someone else's phone number in a mobile phone. It's not unique. It can let others know whose address this address belongs to.

The user name you use when sending a letter is like a caller ID, so that the recipient can know who sent it to him. You can modify this user name, for example, send it to your wife, you can write "husband" and send it to your mother, you can write "son" and so on.