I do not remove anything from the existing subject line, leaving default " Invoice # XXXX from " remains.  

If I hand type in client name after pre-populated text, it remains and works, but I would have to hand type every time.  I need to know field names that will work in this area, as I have tried same field descriptors used within existing QB forms including fields that work perfectly within default Invoice verbiage    ...

Invoice 612593 from XXX   [customer display name]  when email is delivered, it looks exactly like this  it does not populate with client company name or display name.  I have tried several other field descriptors to map to subject line with same result.

All QB templates are outdated, archaic looking, and dysfunctional.