Ideally something your HR or legal department, if you have such, knows how to deal with.
Similar to a staff search policy it needs to be handled correctly - there needs to be a legitimate reason but that can be as simple as retrieving information an employee might have deleted on leaving the company. Best practise would be to have at least 2 management level colleagues present* and ideally the ones dealing with it, anything which is identified as personal and unrelated to work should be secured and/or deleted as appropriate (regardless of what policies the company may or may not have as to the use of work email accounts) and in no circumstances disseminated. There might be some cases where personal emails have implications for the security of the company and/or new or ongoing disciplinary action but that gets into very different territory and needs proper legal advice and can very much backfire on the company if not handled correctly.
* Alternatively having the (ex)employee or a representative present and/or getting their permission first.
Similar to a staff search policy it needs to be handled correctly - there needs to be a legitimate reason but that can be as simple as retrieving information an employee might have deleted on leaving the company. Best practise would be to have at least 2 management level colleagues present* and ideally the ones dealing with it, anything which is identified as personal and unrelated to work should be secured and/or deleted as appropriate (regardless of what policies the company may or may not have as to the use of work email accounts) and in no circumstances disseminated. There might be some cases where personal emails have implications for the security of the company and/or new or ongoing disciplinary action but that gets into very different territory and needs proper legal advice and can very much backfire on the company if not handled correctly.
* Alternatively having the (ex)employee or a representative present and/or getting their permission first.