Your employer has a duty to inform you if they are planning to monitor email in your workplace, telling you when and why it is being carried out and who will have access to your emails.
Simply telling you that, for example, your emails may be monitored is not sufficient. You should be left with a clear understanding of when information about you is likely to be obtained, why it is being obtained and how it will be used.
If you have started in a new organisation, check for details of this in any staff handbook that you receive, or can readily access. Workers should be reminded of existing monitoring periodically, and when new monitoring arrangements are introduced, they should be told.
Some employers build in an automatically generated reminder as to the presence of monitoring equipment that flashes up every time the computer is switched on.