An employer is entitled to access your inbox or voicemail if they believe that there will be business communications that need to be dealt with in your absence, and if they have warned you clearly in advance (for example in an internet policy) that this might happen whilst you are away.
Employers are also entitled to do this if they suspect serious wrongdoing such as fraud.
Employers must do their best to avoid accessing communications that can be positively identified as not relating to the business without opening them (e.g. emails marked 'Personal' in the subject line). Employers who read obviously private messages without their employees’ clear and freely given consent risk breaching the General Data Protection Regulation and the Regulation of Investigatory Powers Act 2000.
It is sensible to clearly mark the subject line of any personal emails with the word “Personal”.
Alternatively, to minimise risk of your private or union emails being intercepted and read, it is best to avoid using a work email or voicemail system for private or union matters.
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