Simple question that support will not answer for me, exactly what part of the email header populates the "web email" field when you send an email to case? 

I need to build a case and sometimes link to a contact (works great) and sometimes only link with an account. When i try and link to just teh account i get

ErrorMessage: Case: bad field names on insert/update call: AccountId
Fields: AccountId
StatusCode: INVALID_FIELD_FOR_INSERT_UPDATE

 I must be missing somethign simple, what is it?