The use of e-mail and messaging in the workplace has become a common work activity, not only as an instrument for the undertaking of workplace activity, but also as a means of monitoring and controlling of workers, so that often the content of electronic mails is presented in court as proof of violations against workplace rules and to justify the sanction imposed. However, the provision of information obtained through email or messaging as evidence in the labour process requires the prior qualification that the evidence was obtained in a lawful manner. In this chapter, we analyse the use of electronic mail and messaging in the development of work activity as a way of controlling of such activity. On the one hand, we look at the compatibility of those means of surveillance and supervision of labour activity with the constitutional right to privacy, in order to verify the legality or illegality of the evidence obtained and, on the other hand, we examine the probative effectiveness of the information obtained and how it is incorporated into the labour process, normally in order to support disciplinary action.