Hello Piacas,
Can you post the used LDAP filter?
A few possible root causes:
Your filter excludes (or does not include) the needed object class for your mail enabled groups (the objectclass for group object = "group")
There is another attribute missing (or present) that is mandatory (or should not have any value)
The BaseDN you use us excluding the container that contains your mail enabled groups.
Hope this helps.
Steven