Dovecot configuration: Home dir not found

Hi, I've my mail solution configured on Server-A with webmail, Dovecot and Postfix. I'm trying to deploy only Dovecot and postfix on Server-B to reduce IMAP/POP3/SMTP load on server-A. Please note server-B does not mailboxes. It should be mapped to mailboxes on server-A. While the LDAP query from Server-B to Server-A is going successfully, the imap service on Server-B is throwing the below error.

imap(user@example.com): Debug: Home dir not found: /mail/vmail/**/

I guess imap is looking for *home dir* in server-B. How do I tel the imap that home dir is on server-A and not server-B.

Please let me know if there exists a configuration parameter for the above..

Thanks Divakar