Authenticate users using their firstname

Hi,

I'm setting up a Postfic and Dovecot with LDAP email server. My users in LDAP is like this:

dn: uid=firstname,ou=People,dc=domain,dc=com
uid: firstname
uidNumber: 4025
gidNumber: 4025
givenName: firstname
objectClass: top
objectClass: person
objectClass: posixAccount
objectClass: shadowAccount
objectClass: organizationalPerson
objectClass: inetOrgPerson
loginShell: /bin/bash
homeDirectory: /home/firstname
cn: firstname lastname
mail: firstname.lastname@domain.com<mailto:firstname.lastname@domain.com>

This is how I connect Dovecot with LDAP

hosts = ldapserver
ldap_version = 3
base = ou=People,dc=domain,dc=com
deref = never
scope = subtree
user_attrs =
user_filter = (&(objectclass=inetOrgPerson)(uid=%n)
pass_attrs = uid=user,userPassword=password
pass_filter = (&(objectclass=inetOrgPerson)(uid=%n))
default_pass_scheme = SSHA

When I enter a user's email address and password as the following: email: firstname.lastname@domain.commailto:firstname.lastname@domain.com password: password

and according to my setting which I used "%n" as you see above, the username used to authenticate is "firstname.lastname". I checked the Dovecot variables but I couldn't find something useful in this case to manipulate the "%n" variable.

I would like to keep using email addresses as "firstname.lastname@domain.com"mailto:firstname.lastname@domain.com but authenticate users using their first name. I really hit a wall here and any help will be much appreciated.