I might be wrong about this, and if I am, I hope someone will correct me and give us the most up-to-date answer.
Even if a subscribing member does contact you (a non-subscribing member,) it does not activate all subscriber privileges for the non-subscriber.
The non-subscriber can only make PM replies to the subscribing person who initiates a PM exchange; the non-subscriber cannot then go on to PM whomever they want like a subscribing member can.
However, I could be wrong about this.
I'm hoping
@Oncefallen or whoever knows about this can clarify.
Thank you to whomever can answer this!