I'm working forward from a stock install of 6.5.1. I checked both the language files for messhandler and messreply - there's simply nothing there that says something along the lines of "This using isn't accepting private messages, so you can't reply to them." There's one for if they're ignoring you, and there's a "NOPRIVATE" check in the sendmessage.php, but there's just not a check in messreply.php for this case.