Ah yes, updates that should take one or two hours can in fact take over three days. Despite confirmation emails in the meanwhile saying the mail account is setup and ready for use.
I'd guess this is down to what one might politely describe as "historical reasons". Users have been requesting it be sorted out for quite some time, and if I recall correctly DH have said it is on their to-do list.
You do have an option of avoiding the numbered mailbox usernames - create your mailboxes not on Mail, Manage Addresses box instead on Users, Manage Users. Still you don't get a free choice of user name i.e. local to your domain, but you can have a alphabetic name, provided it is not already taken.