dovecot-ldap.conf user_attrs = mail,/var/spool/virtualmail/%d/%n,,,,

Sorry, I posted an answer to your later question which I think is useless given this question. These user_attrs need to be LDAP attributes like:

User attributes are given in LDAP-name=dovecot-internal-name list. The

internal names are:

home: Home directory

mail: MAIL environment

system_user: System user name (for getting user's groups from /etc/group)

- For virtual users you don't want to use this, so this defaults to none.

uid: System user ID

gid: System group ID

user_attrs = mail=user,mailMessageStore=home,,,

reference: http://wiki.dovecot.org/HowTo/DovecotOpenLdap http://wiki.dovecot.org/UserDatabase/ExtraFields