Maximum amount of mailboxes

[ledouxe]

Hi,
A client of mine has come upon what seems to be the limit of mailboxes 
for one letter. They use student numbers for mailbox names. When a 
mailbox is numerical, Cyrus puts the mailboxes under the letter "q" in 
/var/spool/imap (ex: /var/spool/imap/q/user/12345/ )
My client can't create any more mailboxes (he has around 40 000). If he 
deletes one, he can create one but can't create any more. But he can 
create a new mailbox named "foo" for example. Just no more numerical 
mailboxes.

Has he reached Cyrus' maximum for one letter? Am I looking in the right 
place?

Thank you.