[Dovecot] Set only INDEX path

[Kilian CAVALOTTI]

Hi folks,

I'd like to know if I can set the INDEX path in the default_mail_env, 
without setting the format and location of mailboxes (something like 
default_mail_env = :INDEX=/var/mail/indexes/%u)

I want to let each user choose whether he prefers to store his mail in mbox 
or maildir format, so I don't want to /force/ the configuration in the 
default_mail_env variable, but let the autodetection feature of Dovecot do 
the job.

The problem is that home folders are NFS mounted on the mail server. So I'd 
like to put the indexes somewhere locally on the mail server.

I tried to set the default_mail_env with various combinations of ':' and 
INDEX, but with no success, I invariably get 
"Fatal: Failed to create storage with data..." error messages.

Is there any solution to my problem ? ;)
TIA,

-- 
Kilian