The m9999999 number is just the mailbox username which is completely transparent to sender and receiver, it’s only used to verify your identity to the mail server. So my question would be, why does it matter to you? You can specify your name and address/reply address in the mail client.
As for using a name, you might try setting up a new user. New users get a mailbox automatically, and I think you can use the name instead of the m999999 as the username to retrieve mail.