Thanks, I always wondered why on some systems Mail seemed to ignore the IMAP prefix!

Thanks!

It depends on the IMAP server software in use on the server. My server used to use a program called courier-imap, which presents all other folders as children of "INBOX", with each segment separated by "." (like how a unix system uses "/" to separate directories and filenames.) If I were to manually configure Mail.app to work with this system, the "namespace prefix" would be "INBOX.".

I've since switched to dovecot, which can be configured to use any number of different namespace layouts- one of which is the one used by courier-imap, which made the transition easy (i.e. transparent) to my users.

The IMAP protocol itself allows the server to tell the client what to use as a namespace prefix. Both courier-imap and dovecot do this; it sounds like the server you're using is not.