Compare EntityReference with Entity

Hi... I'm new in CRM...

I have an Email entity containing an attribute CreateBy with a reference to entity SystemUserSet.
Now I want to  get email that create by spesific user.

I have try this query:  "var we = from a in crm.EmailSet where a.CreatedBy.Equals(from x in crm.SystemUserSet where x.LastName == "Ibnu Khakim" select x) select a;" but failed.

Anyone can help me...