An employer is entitled to access your in-box or voicemail if they believe that there will be business communications that need to be dealt with in your absence, and if they have notified you that this will happen whilst you are away.
Employers should take all possible steps to avoid accessing communications that can be positively identified as not relating to the business without opening them, e.g. e-mails that are marked 'Personal' in the header.
To be more assured of privacy in such a situation, we would recommend starting personal e-mails' subject lines with the word 'Personal' and any confidential e-mails relating to union matters with the word 'Union'.
Bear in mind though that this will not guarantee any level of privacy if the employer has another reason to intercept your email (for example if they suspect criminal activity, or they need to check that the business is complying with legal obligations).
