using LDAP for email aliases

Dear All,

We use /etc/aliases on our mail server so users can receive email addressd to several accounts;

e.g. users have a short account name e.g. "joeb"
then a firstname.lastname, such as "joe.blogs"
and then might belong to a group such as "unixdev"

the /etc/aliases file would look something like this:
joe.blogs: joeb
unixdev: joeb, katec, lisad

PMG isn't aware that joe.blogs and unixdev are aliases, so the users receive a SPAM report for each alias - it would be nicer if they received just one.

From reading other posts here I understand the solution is to use LDAP.
Great, our PMG install uses OpenLDAP for authenticating logins, but, we don't currently store email aliases in our directory.

To add aliases to our directory, should we be using "objectClass: nisMailAlias" or "objectClass: alias" ?

An chance someone could share an LDIF showing how the alias data is stored...?

many thanks

Jake