You generally need to convince the airline you're allowed into the destination country, because if you're stopped at the border, they're responsible for you. I

You generally need to convince the airline you're allowed into the destination country, because if you're stopped at the border, they're responsible for you. In many -- probably most -- countries there is some kind of outgoing border control for any international flight -- not necessarily "customs" but something similar. (I was actually detained briefly at customs leaving Sydney for Japan in the 90s. Never found out why.)