Please give me some advice. Can java be used as a mailbox?

I don't know what you want to do with the mailbox either. Is it a mail server? Or the client? A complete mailbox is not that simple. You don't even know what to do. Of course java can do it. Every language can do it. The apache website has java open source mail service system (james), and there are many clients on the Internet. You can have a look at the java open source encyclopedia.