Ok that does not automatically give you right to access is emails – even if someone using a works computer they still have a right to privacy so you are only able to access these private emails if you had a very clear and specific policy allowing you to do so. There should have been something in his contract or something that is a separate policy and referred to in his contract which allows you to monitor his email use to be able to access it and then use t against him as evidence. So you need to be very careful about using this as a basis for disciplinary action and if there is no such policy in place, then I advise against it.
When it comes to stress in the workplace, as an employer you have a duty to try and deal with that and to remove or minimise any stressors which may be present and affecting the employee. You are only required to do what is reasonably expected of an employer, rather than what the employee may be asking for, so an objective approach is generally acceptable.
If he has raised grievances but is unable to attend a grievance meeting then you can see if he is happy to have the grievance dealt with in his absence, or via other means such as over the phone or in writing.
If he is going to be absent for the short to medium term then you may have to just allow him that time off if he is signed off sick. If he is going to be absent long-term then you can consider instigating the capability procedure, which could eventually result in a dismissal. This would be a case of you stating that he is no longer capable of performing his job as a result of these long term absences.
This is your basic legal position. I have more detailed advice for you in terms of the law on capability procedures and what is expected of you, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there is no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you