In my GroupOffice installation I've got several mailaccounts using the same external mailserver. Most of these accounts are autocreated by imapauth module during the first login of the correlating user. Some are manually created as additional mailaccounts. For every account mbroot is set to 'INBOX' (even in imapauth.config.php the parameter 'mbroot' => 'INBOX').
The manually created accounts work like a charm and all folders are shown in correct structure. Mbroot stays 'INBOX'.
For the autocreated accounts the parameter mbroot is cleared everytime the assigned user logs in. No matter if mbroot is set to 'INBOX' manually in GroupOffice, directly in database or not. The affected accounts work in mail module indeed, but all folders are shown as subfolders of INBOX.