isn't that why you would have several options in your NIP-65 in/outbox advertisment? also, you know, if the clients would support NIP-42 (does gossip btw?) reading from someone's outbox when one is the addressed recipient should be permissible and could only be gated as such by using NIP-42 implement NIP-42, that's to all client devs, not just you (you know i'm not using yours because i can't test with it)
NIP-42 isn't the only gate. Relays may want you to pay them some sats, or resubmit an event with a minimum PoW, or complete a CAPTCHA, or any number of other crazy anti-spam schemes that have yet to be invented. Yes gossip does NIP-42. It is nearly essential to nostr at this point.