Hmm. This I don't have. On the Mail>Addresses panel, only the two "m#######" mailboxes I have created show in the mailboxes list. The "username" mailbox definitely exists, as it shows up in the "Filter and Spam" panel, and I can add filters to it. I can also log in to this mailbox from my email client, synchronise it (I use IMAP), and send email FROM it.
But I can't find out what email address directs TO it, or how to create a new one that does.