PMs stuck in Outbox

I don’t know why, but 2/3 of messages I’m trying to send goes to Outbox and never leaves it. I’ve replied for 3 messages today, and 2 are stuck in there. Am I doing it wrong or what’s the issue?

Almost definitely the receiver simply hasn’t opened it yet. A lot of new users IME post ads and then don’t check in very often. They may have sold the product or whatever already.

Oh, I wasn’t aware that the receiver need to be online to get the PM. Thanks, I hope then, they will get those soon.