Notificaciones electrónicas are the official communications that the AEAT (Spanish Tax Agency) and other public bodies place in a digital mailbox, the DEHu, instead of sending you a paper letter; for companies the system is mandatory. The detail that matters: if you do not open a notification, after 10 calendar days it counts as rejected and every deadline keeps running exactly the same.
The routine that prevents nasty surprises: check the mailbox at least once a week, switch on the courtesy email alerts (they do not replace the notification itself), and keep your digital certificate valid. An unread requerimiento does not go away: its clock keeps ticking.
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