Poland’s data protection authority, the UODO, offers guidance on email monitoring in the workplace:

• The employer may introduce monitoring of the employee’s e-mail when it is necessary in the employer’s opinion to ensure work organization that allows full use of working time and proper use of the work tools provided to the employee.
• The monitoring may not violate the secrecy of correspondence and other personal rights of the employee.
• You must inform the employee for what purpose you intend to use monitoring, as well as the scope and method of monitoring. This information is determined in the collective labor agreement or in the work regulations or in a notice, if the employer is not covered by the collective labor agreement or is not obliged to establish work regulations.
• You should inform employees about the introduction of monitoring. This can be by a letter addressed to each employee, an announcement on the bulletin board, an e-mail message addressed to the staff or information provided via the company’s intranet.
• You should inform the employee about the introduction of e-mail monitoring no later than two weeks before its launch.

Details from the Polish Data Protection Authority.

[View source.]