Apparently reply-by-email isn't set up yet. Judging from the comments in the file notifications/tcl/notifications-security-procs.tcl that is propably responsible for dealing with incoming replies it wouldn't be a good idea to activate this at a public site now, because the security checks necessary to make sure that a reply really came from the user that s/he claims to be are not implemented yet. Or maybe the code in that file should serve another purpose and I'm missing something, but there are procs like this in it:
ad_proc -public can_notify_object_p {
{-user_id ""}
{-object_id:required}
} {
# hack
return 1
}
