You could chain these lookups (and be more creative with regards to how you do them) to do whatever you want. For instance, you could do the following with the lookup chain:
LiveLDAP -> Custom Script -> CSV Lookup
The LiveLDAP lookup will return a value for the user's "Department" attribute if you define that.
Then the script could perform logic that says "if the department = 'A', then make csvlookupkey = sender-email, else make csvlookupkey = null.
Then if you define the key value in the CSV lookup as the attribute 'csvlookupkey', you would only get a result against that CSV lookup when the user's department was = 'A' (or whatever you wanted to do with that logic in the script).
In short...it CAN be done with DLP, you just have to know how to do it. Granted, in much more complex cases it might be better done with Workflow, but this one is pretty easy.
~Keith