For reasons of confidentiality and to avoid claims of misuse of company time and property, you are advised to make such communications in your own time, using your own equipment and personal email account. You should never assume that email is confidential and it is sensible to exercise caution in terms of the information you send over the internet.
Employers commonly have policies designed to restrict the use of company email systems. You should look in the staff handbook for the terms of the IT Equipment Policy where you work.
Some of your emails should not be accessed by your employer, even if you send them using a work email. For example, your employer should not open emails that are clearly marked as private, or emails where you were preparing for a disciplinary or grievance hearing and consulting with the person who will be accompanying you. But even with emails clearly marked, say, 'Personal' or 'Union business', in practice, there is always a risk that the communication will be intercepted, so it is 'better to be safe than sorry'.