I have discovered last month (thanks to this post) that you can send postcards from Spain using Swiss Post service instead of the normal Spanish post service, named Correos.
This is how it works: In many souvenir shops, kiosks, hotels and camping sites you can buy ”Swiss Post stickers” that can be used to mail postcards from Spain to anywhere abroad. Postcards bearing ”Swiss Post stickers” must be placed in a ”Swiss Post mail box”, which you can find at the sales points. The ”Swiss Post mail boxes” are yellow and bear a ”Swiss Post” logo accompanied by a red-rimmed cross. Swiss Post will then collect and dispatch your postcards.
Why use Swiss Post? “Swiss Post stickers” are widely available where tourists usually buy their postcards. Swiss Post currently has sales partners on the Balearic Islands, on the Canary Islands, in Alicante, in Huelva and will soon be present in Malaga and Catalonia too. Cooperating with the French La Poste in their joint venture company ASENDIA, Swiss Post is able to use reliable delivery channels worldwide. On average, postcards are delivered within 7-8 days in Europe and within 8-9 days overseas. The service is available at a similar price to Correos (Spanish post).
(Taken from here)
In this video you can see both mail boxes together (I haven't seen them so far). Isn't it a bit confusing? Especially, if it is intended for tourists...
Do you have more than one postal service in your country?