As far as I know most countries do or at least did in fact refuse to deliver mail addressed to "Crimea, Russia". In order for it to get through, you either need

As far as I know most countries do or at least did in fact refuse to deliver mail addressed to "Crimea, Russia". In order for it to get through, you either need someone to bounce it within Russia or to disguise it from your national post office but not from the Russian post office (no doubt this is easy: just omitting "Crimea" or using the Cyrillic alphabet would probably work). In the same way, they refuse to accept a Russian passport issued to someone in connection with Crimea. These are not recognised as purely domestic matters; the statehood is intimately connected with the territory it controls.