LAW 39/2015:
Article 41. General conditions for the practice of notifications .
"1. Notifications must preferably be made by electronic means and, in any case, when the interested party is obliged to receive them by this means.
However, administrations can practice notifications by non-electronic means in the following cases:
a) When the notification is made on the occasion of the spontaneous appearance of the interested party or his representative at the registration assistance offices and requests communication or personal notification at that time. [...]
By regulation, administrations can establish the obligation to practice electronic notifications for certain procedures and for certain groups of natural persons who, due to their economic, technical capacity, professional dedication or other reasons, are accredited to have access and availability of the necessary electronic media."
Article 44. Unsuccessful notification .
"When those interested in a procedure are unknown, the place of notification is ignored or, once this has been attempted, it has not been possible to practice, the notification must be made by means of a published announcement in the "Official Gazette of the State". Likewise, previously and on an optional basis, the administrations can publish an announcement in the official bulletin of the autonomous community or province, on the notice board of the City Council of the last address of the person concerned or of the consulate or section consular of the corresponding embassy. The public administrations can establish other complementary forms of notification through the rest of the media, which do not exclude the obligation to publish the corresponding announcement in the "Official Gazette of the State".