consider tuning the text "mailbox must be installed" for example to "mailbox must have been installed and created" when user starts to create email addresses.
Why?
The following happened:
(1) created mail user x
(2) tried to create email address y to user x
(3) failed, user x was not in the list to pick
(4) checked all mail users, x was in the list
repeated (2) ... (4) without succeeding
took coffee break
(5) message "mail user x created" on display
succeeded in (2)
Obviously there is delay before the mail user is ready to be used for email address creation and therefore server software should display somethink like "you must install mail user and also wait until everything has been created before you start using it for email addresses"
Note that when the server lists domains to be picked for the email address, you really can see all the domains created whereas when it lists mail users for picking one it may not be in the list allthough you have created it.
