This has likely been suggested already, but it would be very helpful to have PMs to which I've already replied marked as such.   Much easier to see at a glance, rather than cross-checking my Inbox with my Sent box.    
   
 Yeah, I can place "replied to" messages into a separate folder or delete them after replying, but this would be preferable to me.    
   
   
 I'm sure there's a flaw somewhere, but it's just an idea.    
   
   
   
 xoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxoxo