Messaging clients
Your inbox, what the composer does, calls, and the one-thread rule.
Messaging is free for you, and always will be. What is behind a subscription is the client's ability to start one.
One conversation per client
You and a given client have exactly one conversation, whatever brought you together. Applying to a second job of theirs moves that conversation's context rather than opening a second thread with the same name in your list.
Who can start one
A subscribed client, from your profile or from your application. You can reply to anybody who has written to you, always.
You cannot start a conversation with a client who has not written to you. The route in is applying to their job, or being invited.
If your status is not currently available, clients cannot start new conversations with you at all. See availability and hiring status