Because many couples share access to personal email accounts such as gmail, hotmail, or yahoo, some people choose to use their work email as their "private" method of correspondence. Some people also used their work emails for their Ashley Madison accounts (looking at you, certain Government Employees). Not cool.
Now may be a good time to review your company's email policy and refine it to address this growing concern. I always recommend that companies give their employees a work email. As part of this, I also always recommend companies have a policy in place that states work email addresses should be used for work-related purposes only. A good email policy should also state that no employee has a right to privacy when it comes to his or her work email. Period. No exceptions. Not only does this preserve the sanctity of the work email by making it more secure from the outside world, it also encourages employees to keep their personal doings outside of the office.
If you do not have an email policy in place, I would strongly consider implementing one soon. It doesn't need to be a lengthy work of art, it just needs to get to the point: it is a violation of work policy to use work email for personal correspondences and/or personal reasons that are unrelated to one's position with the company. Any violation of this policy should lead to disciplinary action pursuant to the company's disciplinary policy.
If you already have a solid email policy, now may be a good time to remind employees about what it says. There is no need (nor is it appropriate) to search the Ashley Madison database and call out your cheating co-workers, but it may be worthwhile to simply state the obvious: some people may be using work emails for personal reasons and this is in violation of a company policy.
Cyber security is a big deal and this is one of the easiest (and cheapest) ways to protect your company from cyber threats. Give your employees a work email address but also give them rules. #theend