Post offices ( Correos ) are generally found near the center of towns and are normally open from 8am to noon and again from 5 to 7.30pm, though big branches in large cities may have considerably longer hours and usually do not close at midday. Except in the cities there's only one post office in each town, and queues can be long: stamps are also sold at tobacconists (look for the brown and yellow Tabac sign).
You can have letters sent poste restante ( Lista de Correos ) to any Spanish post office: they should be addressed (preferably with the surname underlined and in capitals) to Lista de Correos followed by the name of the town and province. To collect, take along your passport and, if you're expecting mail, ask the clerk to check under all of your names - letters are often to be found filed under first or middle names.
Outbound mail is reasonably reliable, with letters or cards taking around five days to a week to the UK and Europe, a week to ten days to North America, New Zealand and Australia.