If you send the email to an account controlled by the employer or by using the employer's equipment, the employer almost certainly has the power and the right to review anything sent through or by anything or anyone it owns or employs.
Many employers, especially big ones, have automated processes that make several copies of everything sent or stored on their computer systems. Some may possibly even be capturing or storing information sent through their internet connections.
If you use your personal computer or phone and use an email account not controlled by your employer and send a message to another account also not controlled by your employer by using your own internet connection, the employer would not have any means or right to look at your message without your consent or your recipient's consent.
There are exceptions as to when an employer has no legal right to read emails sent by its employee. However, your best practice, if you do not want your employer to read your emails, is not to send emails from or to the computers or other equipment provided by your employer.
As my colleague stated, the answer in most cases is yes, your employer has the right to review emails sent on or received using their system. You should start bey reviewing your employee manual to know the specific rules in your case.
While I cannot give you legal advice in this forum, I believe that your employer can so review the emails. To try to avoid that issue, if you have internet at work, set up a personal yahoo.com or gmail.com email account to use for personal emails. Do not use your work email. That may solve the problem.
THIS RESPONSE IS INTENDED TO CONVEY GENERAL INFORMATION ONLY. IT SHOULD NOT BE RELIED UPON OR TAKEN AS LEGAL ADVICE. FURTHER, THIS RESPONSE IS NOT INTENDED TO AND DOES NOT CREATE AN ATTORNEY-CLIENT RELATIONSHIP.