Dynamic Subject Line

%%[set @firstName = AttributeValue("FirstName")
set @UNvalue= "?" 
set @firstName = ProperCase(@firstName)
if @firstName == "unknown" OR IndexOf(@firstName,@UNvalue) > 0 then set @FirstName = "" endif
if not Empty(@firstName) then set @greeting = Concat(", ", @firstName)
else
set @greeting = ""
endif ]%%