When can I message the other person?

As soon as a booking or rental links you to the other person — but not at the same point in each case. For a flight, once the pilot has accepted your request. For a rental, as soon as you send your request, even before the owner accepts it.

Why this exact moment

There's no open messaging between strangers: you only write to someone you have an active deal with. For a flight, contact opens only after acceptance: before that nothing is captured, and chatting would allow arranging a deal off the platform. For a rental it's the opposite — the owner needs to talk before accepting, to check your experience and the aircraft's availability.

If the request doesn't succeed

A declined or unanswered flight opens no conversation, since contact comes after acceptance. A declined or cancelled rental keeps its messages readable: nothing disappears, you simply can no longer add to it.