They are twiners and it would depend on how wide the mailbox pole is for whether the vines will naturally wrap around it. And if they did, once the vines reach the top, they will still need something to twine around or they will use themselves as a twining support and will eventually fall over in a tangled mess. Because it is such a large vine with large flowers, I don't know if that is a good choice for a mailbox like morning glories might be (which have thinner vines) unless you setup some kind of tee pee behind the box for them to twine on.

A mandevilla might work better on a light pole, although you could consider pruning off the growing tips and try to keep it bushier once it reaches the top of a mailbox.

Thanks jenny for the advice, your balcony looks wonderful. Beautiful plants and colors.