It is likely that your provider has already set up a mailbox on your account. If you are going to use your mail account as a mail forwarder or mailing list, you are recommended to switch off the mailbox service: a mailbox keeps all incoming messages and messages are not removed automatically after being forwarded. Therefore, if you choose to have the "mailbox + mail forwarder" configuration on your account, be sure to clean up the mailbox from time to time.
To set up a mailbox:
Now your mailbox is created.