Slated members can message you if you are tracking them or if you have your score-based messaging permissions enabled and their score is above the limit you set. Score-based messaging permissions are enabled on your account by default.
Changing Your Messaging Permissions
You can adjust your messaging permissions by going to your Settings - Privacy section and choosing an option from the "Who can contact me?" setting. You can expand the people who can message you based on a different score threshold, or allow anyone on Slated to message you. You can also restrict who can message you to only those members whom you track, or to no one at all.