The initial appearance of picture postcards (and the enthusiasm with which the new medium was embraced) raised some legal issues that can be seen as precursors to later controversies over the internet. Picture postcards allowed and encouraged many individuals to send images across national borders, and the legal availability of a postcard image in one country did not guarantee that the card would be considered "proper" in the destination country, or in the intermediate countries that the card would have to pass through.
Some countries might refuse to handle postcards containing sexual references (in seaside postcards) or images of full or partial nudity (for instance, in images of classical statuary or paintings). In response to this new phenomenon, the Ottoman Empire banned the sale or importation of some materials relating to the Islamic prophet Muhammad in 1900.
Affected postcards that were successfully sent through the OE before this date (and are postmarked accordingly) have a high rarity value and are considered valuable by collectors.