In general, email is a very convenient medium for both landlord and tenant and the legality of the use of it has been introduced in the Ley the Arrendamientos from June 2013, in article 4. If both parties have agreed to use it as their means of communication on day-to-day matters and have formally exchanged their email addresses (preferably even mentioned in the contract), all is well.
It is a different matter however, for notifications that concern the rental agreement itself, rent increase, extension or termination for example.
This is what art. 4.6 of the LAU states:
“Las partes podrán señalar una dirección electrónica a los efectos de realizar las notificaciones previstas en esta ley, siempre que se garantice la autenticidad de la comunicación y de su contenido y quede constancia fehaciente de la remisión y recepción íntegras y del momento en que se hicieron”.
Parties can indicate an email address to be used for notifications, provided the authenticity of the communication and its content can be guaranteed, and there is reliable proof of the sending and receipt and the moment either took place.
For the email to be equally valid as a burofax in the eyes of the law, the contract must contain a clause stating explicitly that both parties agreed on email as their chosen means of communication and the email addresses must be included.
To comply with the ´reliable proof of the sending and receipt and the moment either took place´ you can only use email accounts that offer automatic ´acuse de recibo´ (receipt notification). The clause in the contract must state that both parties agree to activate that option.
The clause should further contain a statement that parties allow for an independent expert to do a search on their computer in case of a disagreement about email notifications being sent/received or to verify the authenticity of the content of the emails presented in court. And that a refusal from either side will lead to termination of the contract, with possible indemnization of the damaged party.
Concluding, to comply with the requisites for email notifications to have legal value, is not that simple, as the email providers that are used most, like Hotmail or Gmail, don´t offer the option for automatic notification of receipt.