yapadu,

In LDAP, you can search objects using various attributes, for example by uid or cn. SpamFilter by default will lookup a "username" in the sAMAccountName, the uid, and the UserPrincipalName attributes. The "username" is whatever you decide to use as a key to identify users in your LDAP installation. If you wish to use a different attribute, such as "mail" for example, you can just add:
(mail=%0:s)
to the list of attributes being searched by SpamFilter when performing the LDAP query.

Once SpamFilter locates the user (for which the credentials - username and password - are provided in the SMTP session) in LDAP, it will then try to authenticate such user with the above username/password on your LDAP server. If the authentication request is successful, the user is allowed to send mail.

There are no requirements in how passwords are stored in LDAP. You can use clear text, crypt, MD5, etc. There is no need to use reversible encryption, as SpamFilter is simply using the username/password provided in the SMTP session to attempt an authentication request to LDAP.